Document:

<PAGE>   1
                                                                   EXHIBIT 10.21

                           EXCLUSIVE LISTING AGREEMENT

        THIS EXCLUSIVE LISTING AGREEMENT ("Agreement") is made as of the 21st
day of April, 2000, by and between EBS BUILDING, L.L.C., a Delaware limited
liability company, c/o PricewaterhouseCoopers LLP, 800 Market Street, Suite
1800, St. Louis, Missouri 63101 ("Owner") and COLLIERS TURLEY MARTIN TUCKER,
INC. , a Missouri corporation, 34 North Meramec, Suite 500, Clayton, Missouri
63105 ("Broker").

                                   WITNESSETH:

        WHEREAS, Owner owns the building presently known as One Financial Plaza,
with an address of 500 North Broadway, in St. Louis, Missouri, appurtenant
related parking facilities (partly owned in fee, partly by leasehold) and
appurtenant common areas (hereinafter, together with the real estate on which
the same are situated, collectively referred to as the "Property"); and

        WHEREAS, Owner desires to engage Broker as Owner's agent to lease the
Property and Broker desires to accept such engagement, upon and subject to the
terms and conditions set forth herein.

        NOW, THEREFORE, for and in consideration of the mutual agreements set
forth in this Agreement, and for other good and valuable consideration, the
receipt and sufficiency of which are hereby acknowledged, Owner and Broker
hereby agree as follows:

1.      EXCLUSIVE RIGHT TO LIST

        Owner hereby grants to Broker the exclusive right to obtain tenants for
        vacant space in the Property, including the negotiation of renewals of
        leases, for the term of this Agreement. Notwithstanding anything to the
        contrary herein or elsewhere, all tenants, all terms and conditions of
        all lease proposals and all leases and renewals shall be subject to
        Owner's sole and absolute discretion. Broker shall have no authority to
        extend any offer or make any agreement on behalf of or binding on Owner,
        and Broker shall have no authority to accept security or other deposits
        in connection with lease proposals or leases or renewals; accordingly, a
        lease (or any renewal thereof) shall become effective only when (a)
        signed by an authorized signatory on behalf of Owner and tenant and (b)
        delivered by Owner to such tenant.

2.      TERM

        The term of this Agreement shall end, and this Agreement shall terminate
        and be of no further force and effect, on October 31, 2001; provided,
        however, that (a) Owner and Broker shall each have the right to
        terminate this Agreement and end the term without cause at any time by
        providing thirty (30) day's prior written notice to the other party, (b)
        Owner shall have the further right to terminate this Agreement and end
        the term without cause immediately upon the termination of Owner as a
        limited liability company or termination of the operating agreement
        dated as of September 26, 1997, under which Owner is constituted,

<PAGE>   2

        (c) Owner shall have the further right to terminate this Agreement and
        end the term without cause immediately upon the sale of the Property,
        the sale, merger or consolidation of Owner, or the mortgaging of the
        Property, and (d) Owner and Broker shall each have the right to
        terminate this Agreement and end the term for cause immediately upon
        giving written notice of such cause and such termination to the other
        party.

3.      DUTIES OF BROKER

        Broker accepts the relationship of trust and confidence established
        between Broker and Owner under this Agreement. Broker agrees to take all
        actions reasonably required or helpful in leasing vacant space in the
        Property as promptly as possible, including promotional, marketing and
        lease up activities, using its diligent and best efforts, skill,
        judgment, and abilities to show space available for lease within the
        Property, offer such space for lease, procure tenants for such space,
        cooperate with outside brokers representing such tenants, obtain
        financial and reference information on such tenants, and negotiate term
        sheets and leases of such space with such tenants, in accordance with
        the economics (e.g. full service rates per rentable square foot, minimum
        and maximum lease terms, expense stops or base years, etc.) and other
        terms and conditions as may be directed by Owner from time to time.
        Broker further agrees to take all actions reasonably required to
        effectuate the renewal or extension of any lease during the term of this
        Agreement. Inquiries for any leases, renewals or extensions or other
        agreements for the rental, use or occupancy of the Property shall be
        referred to Broker, and related negotiations shall be handled by or
        under direction of Broker, subject to the approval and review of Owner
        and subject to the participation of legal counsel selected and retained
        by Owner to assist as necessary in preparation and negotiation of leases
        and other documentation, to provide legal advice in connection with
        leasing issues at the Property, and to institute and defend any and all
        legal proceedings associated with leasing at the Property; provided that
        no such legal proceedings or compromises or settlements of such legal
        proceedings shall be undertaken or defended without, in each instance,
        the prior written approval of Owner. Without limiting the generality of
        the foregoing, Broker specifically agrees to (a) provide monthly
        activity reports to Owner on or before the 20th day of each calendar
        month, detailing the status of the leasing progress of the Property, and
        reflecting leases or renewals or extensions out for signature, proposals
        outstanding, prospects shown space within the Property, active
        prospects, the stacking plan for the Property, and other marketing
        activities, and (b) cooperate in the preparation of the Budget and
        revisions to the Budget relating to the ownership and operation of the
        Property. Notwithstanding anything to the contrary herein or elsewhere,
        Broker shall perform its duties under this Agreement at its own expense
        and shall be entitled to no reimbursement of costs other than as
        expressly provided in Sections 4, 5 and 6 of this Agreement.

4.      COMMISSION ON LEASE

        Upon the lease of vacant space in the Property pursuant to a written
        lease executed during the term of this Agreement by a tenant procured by
        Broker, Owner agrees to pay to Broker a commission in accordance with
        Exhibit A attached hereto and incorporated herein by reference. The
        commission shall be due and payable in two equal installments: (a) fifty

                                       2

<PAGE>   3

        percent (50%) upon the last to occur of (i) the execution of the lease
        by all parties, (ii) the deposit with Owner of the security deposit, if
        any, required under the lease, or (iii) the satisfaction or waiver of
        the last of the contingencies, if any, to which the lease may be
        subject, and (b) fifty percent (50%) upon the last to occur of (i) the
        commencement date of the lease, (ii) the date the tenant accepts
        occupancy of the leased premises, or (iii) the first date on which rent
        is actually paid if the rent commencement date shall be later than the
        commencement of the term. If a cooperating broker has participated in
        procuring a lease, Broker shall be responsible for paying any and all
        commissions to the cooperating broker after Broker is paid by Owner, and
        Broker shall indemnify and hold Owner harmless from and against the
        claims of any cooperating broker with claims arising from a cooperating
        broker relationship initiated by or through Broker including, but not
        limited to, attorneys' fees and expenses in connection with Owner's
        defense against such claims. Notwithstanding anything to the contrary
        herein or elsewhere, no commission shall be owing to or claimed by
        Broker (a) in the event any tenant of the Property, or any person or
        entity claiming by, through or under any tenant of the Property,
        exercises any option or right to renew or extend the term of a lease
        unless (i) said renewal or extension is of a lease entered into prior to
        the effective date of this Agreement and said renewal or extension
        requires Broker to negotiate and/or calculate the rent due for the
        renewal or extension period and Owner is not obligated to pay any other
        listing broker a commission in connection with the renewal or extension
        of said lease; or (ii) said renewal or extension is of a lease procured
        by Broker and said renewal or extension is exercised during the term of
        this Agreement; or (b) in the event Owner is obligated to pay a
        commission to Insignia/ESG, Inc. in connection with the procurement of
        such lease pursuant to the terms of that certain Commercial Property
        Management and Leasing Agreement dated December 31, 1997, as amended, a
        copy of the First Amendment to which is attached hereto as Exhibit B
        (the "First Amendment to Property Management Agreement").

5.      POST-TERMINATION COMMISSIONS

        Within ten (10) days following the termination of this Agreement, Broker
        may submit to Owner a written list of any person or entity proposed by
        Broker during the term of this Agreement as a tenant of any vacant space
        in the Property, who actually visited the Property during the term of
        this Agreement with the intention of leasing space and whose proposal
        for such lease was approved in writing by Owner during the term of this
        Agreement. Broker's list shall include the names and addresses of such
        prospective tenants, and shall identify the vacant space to be leased,
        the terms of such lease, the date(s) of the visit(s), and the date of
        Owner's approval of the proposal for such lease. Owner may dispute the
        validity of the information on such list. Broker shall have no claim of
        commission (and hereby waives any claim of commission) with respect to
        any person or entity procured or claimed to have been procured by Broker
        who shall lease any space within the Property after the termination of
        this Agreement unless and except to the extent such person or entity
        shall be included on such list so submitted to Owner within ten (10)
        days after termination of this Agreement. If either: (a) within ninety
        (90) days from the date of termination of this Agreement (the "Initial
        Period"), Owner (but not any successor in interest to Owner) enters into
        a lease for vacant space in the Property with any such listed person or
        entity; or (b) within the period commencing on the ninety first (91st)
        day and ending on the one hundred

                                       3

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        eightieth (180th) day after the termination of this Agreement, Owner
        (but not any successor in interest to Owner) enters into a lease for
        vacant space in the Property with any such listed person or entity and
        both (i) the principal terms of such lease were previously agreed to in
        writing by Owner and Broker prior to the end of the Initial Period and
        (ii) the terms of such lease substantially conform to the terms agreed
        to by Owner and Broker during the Initial Period, then unless and except
        to the extent Owner shall dispute the inclusion of such person or entity
        on the list so submitted by Broker, Owner shall pay Broker the
        commission in accordance with Exhibit A. The agreements of this Section
        5 shall survive the termination of this Agreement.

6.      ADVERTISING AND MARKETING

        Broker shall (a) advertise the Property and available space within the
        Property; (b) prepare a marketing plan, brochures and collateral
        materials for the Property; (c) engage with a competent architectural
        firm approved by Owner to provide computer-aided drawings (CAD) of the
        rentable areas of the Property and space planning services for
        prospective tenants; and (d) engage competent consultants (e.g. public
        relations, advertising, etc.) on an "as needed" basis. The content and
        extent of Broker's advertising and marketing activities and the services
        and consultants engaged by Broker in connection with the same shall be
        subject to the prior approval of Owner. Broker's identification signage,
        if any, within the Property shall be subject to the prior approval of
        Owner, and Broker shall be responsible for assuming that any such
        signage conforms to applicable laws and code and all requirements and
        restrictions of the leases of the Property and any recorded instruments
        encumbering the Property. Upon termination of this Agreement (or sooner,
        if directed by Owner), Broker shall remove its identification signage
        from the Property and fully restore and repair all damage caused by
        installation or removal; and the agreement to do so under this Section 6
        shall survive the termination of this Agreement. Broker shall be
        responsible for the costs of all marketing, advertising and leasing
        activities with respect to the Property, provided, however, that Owner
        shall reimburse Broker for all such costs and expenses for which Broker
        has obtained the prior written approval of Owner, regardless of whether
        said amounts are included in the Budget prepared by Broker pursuant to
        Section 3 hereof, and excepting, however, that Owner shall pay the cost
        of legal counsel selected and retained by Owner in accordance with
        Section 4 of this Agreement.

7.      NONDISCRIMINATION

        The Property shall be offered without regard to race, color, creed,
        religion, national origin, sex, handicap, or familial status.

8.      REPRESENTATIONS

        Owner makes no representations or warranties regarding the Property or
        the condition of the Property including, but not limited to, the
        presence or absence of asbestos, PCB transformers, or toxic, hazardous
        or contaminated substances in, on or about the Property. Broker is not
        authorized to make any representations or warranties relating to the
        Property unless and except to the extent expressly approved in writing
        by Owner.

                                       4

<PAGE>   5

9.      INDEMNIFICATION

        Owner shall hold harmless and indemnify Broker against any liability,
        loss or expense incurred by Broker as a result of any misrepresentation
        of Owner or its agents (other than Broker) or the negligence or willful
        misconduct of Owner or its agents (other than Broker). Broker shall hold
        harmless and indemnify Owner against any liability, loss or expense
        incurred by Owner as a result of any misrepresentation of Broker, its
        agents, employees, or the negligence or willful misconduct of Broker,
        its agents, employees, or invitees. The agreements of this Section 9
        shall survive the termination of this Agreement

10.     STANDARDS

        Broker shall further the interests of Owner in accordance with Owner's
        from time to time requirements, procedures and directions, in accordance
        with the highest professional standards. Broker shall comply with all
        national, federal, state, and municipal laws, regulations, codes,
        ordinances, and orders applicable to Broker and the activities of Broker
        under and in connection with this Agreement.

11.     ASSIGNMENT

        Broker shall not be entitled to assign any rights or to delegate any
        duties or obligations under this Agreement, and any assignment in
        violation of this prohibition shall be void.

12.     GOVERNING LAW

        This Agreement shall be construed according to and governed by the laws
        of the State of Missouri.

13.     ATTORNEY'S FEES

        In connection with any controversy arising out of this Agreement, the
        prevailing party shall be entitled to recover, in addition to costs,
        damages or other relief, its attorney's fees and costs incurred.

14.     MODIFICATIONS

        This Agreement may not be amended, modified or changed, nor shall any
        waiver of any provisions hereof be effective, except only by an
        instrument in writing and signed by the party against whom enforcement
        of any waiver, amendment, change, modification or discharge is sought.

15.     NOTICES

        All notices or other communications required or desired to be given
        hereunder shall be in writing and shall be effective for all purposes if
        hand delivered to the parties at the

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        respective addresses set forth above (or such other addresses as the
        parties may hereafter designate in writing) or sent to the respective
        parties at such addresses by (a) certified or registered United States
        mail, postage prepaid, (b) expedited prepaid delivery service, either
        commercial or United States Postal Service, with proof of attempted
        delivery, or (c) by telecopier (with answer back acknowledged and so
        long as the original of said telecopy is mailed the next day). A notice
        shall be deemed to have been given: in the case of hand delivery, at the
        time of delivery; or in the case of mail delivery three (3) days after
        having been deposited in the United States mail postage prepaid and
        properly addressed; or in the case of notice sent via overnight delivery
        service or telecopy, upon receipt.

16.     INDEPENDENT CONTRACTOR

        It is expressly understood and agreed that Broker, as an agent of Owner,
        will act as an independent contractor in the performance of this
        Agreement. The parties hereby agree that nothing in the Agreement shall
        be intended or construed to create an employer-employee relationship, a
        partnership, or a joint venture with respect to the Property or
        otherwise.

17.     SUBORDINATION

        This Agreement shall be subject and subordinate to any mortgage or deed
        of trust now or hereafter encumbering the Property or any part thereof,
        and at the option of the beneficiary of any such mortgage or deed of
        trust or any transferee of title to the Property by virtue of
        foreclosure of the lien of such mortgage or deed of trust (such option
        to be exercised by written notice to Broker) or by transfer in lieu of
        foreclosure (i) Broker shall attorn to and recognize such beneficiary or
        transferee as the successor to Owner of the Property, and Broker shall
        thereupon continue to perform Broker's covenants hereunder or (ii) such
        beneficiary or transferee may terminate this Agreement as of the date of
        notice of foreclosure or transfer. Broker hereby agrees that the
        beneficiary of any such mortgage or deed of trust and the transferee of
        title to the Property shall be intended third party beneficiaries of
        this Agreement entitled to enforce these provisions.

18.     LIENS

        Broker agrees to execute and deliver to Owner, as a condition to any
        payment due to Broker from Owner hereunder, waivers of lien pursuant to
        any statute which may grant Broker the right to a lien on the Property
        as may be requested by Owner in connection with such payment. If Broker
        is to pay any cooperating broker out of funds paid to Broker by Owner,
        Broker shall make no such payments without first obtaining similar
        waivers of lien from such cooperating broker, and the delivery of such
        waivers of lien from such cooperating broker shall, if requested by
        Owner, be a further condition to payment due to Broker. Broker further
        agrees that any and all rights which Broker, and anyone claiming by,
        through or under Broker, may have to a lien under any statute shall be
        at all times subject and subordinate to the lien of any mortgage or deed
        of trust, and Broker further agrees, upon Owner's request, to execute
        and deliver to the holder of any mortgage or deed of trust against the
        Property or any interest therein a subordination agreement expressly

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        subordinating any and all such liens rights to the lien of such mortgage
        or deed of trust. The agreements of this Section 18 shall survive the
        termination of this Agreement.

19.     EXCULPATION

        Recourse for the obligations of Owner under this Agreement shall be
        limited to Owner's interest in the Property. The obligations of Owner
        under this Agreement shall not be personally binding on or enforceable
        against Owner, nor shall any resort be had to the assets of any of the
        members, managers, agents, or asset manager of Owner, as the case may
        be, or any of their representatives, partners, members, beneficiaries,
        stockholders, employees or agents.

20.     BINDING EFFECT

        This Agreement shall be binding upon, and shall inure to the benefit of,
        the successors and assigns of the parties hereto, subject to the
        restrictions and limitations of Sections 12 and 19 of this Agreement.

21.     EFFECTIVENESS

        This Agreement shall be effective only upon the effectiveness of the
        First Amendment to Property Management Agreement.

        IN WITNESS WHEREOF, the parties hereto have duly entered into this
Agreement as of the day and year first above written.

OWNER:

EBS BUILDING, L.L.C.

By:      PricewaterhouseCoopers LLP,
         its Manager

         By:   /s/ Keith F. Cooper
            -------------------------
            Name:  Keith F. Cooper
                 --------------------
            Title:  Partner
                  -------------------
BROKER:

COLLIERS TURLEY MARTIN TUCKER, INC.

By:   /s/ Thomas E. Murray
   ----------------------------------
     Name:   Thomas E. Murray
          ---------------------------
     Title:   Senior Vice President
           --------------------------

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                                    EXHIBIT A

                             SCHEDULE OF COMMISSIONS

A.      BROKER ONLY; CO-BROKER NOT INVOLVED:

        DEAL TYPE - NEW LEASE:
        Broker shall be paid a commission of 5% of gross rentals to be paid by
        the tenant during years 1 - 5 of the initial term of the lease and 3%
        thereafter.

        DEAL TYPE - RENEWAL:
        Broker shall be paid a commission of 2% of gross rentals to be paid by
        the tenant during any renewal term(s) of the lease becoming effective
        during the term of this Agreement, except to the extent Owner is
        obligated to pay any other listing broker a commission.

B.      BROKER WITH CO-BROKER INVOLVED:

        DEAL TYPE - NEW LEASE:
        Broker shall be paid a commission of 2.75% of gross rentals to be paid
        by the tenant during years 1 - 5 of the initial term of the lease and
        1.65% thereafter. In addition, the Co-Broker shall be paid a commission
        at the rate of 2.25% for years 1 - 5 and 1.35% thereafter.

        DEAL TYPE - RENEWAL:
        Broker shall be paid a commission of 2% of gross rentals to be paid by
        the tenant during any renewal term(s) of the lease becoming effective
        during the term of this Agreement, except to the extent Owner is
        obligated to pay any other listing broker a commission. Any fees to be
        paid to a Co-Broker for a renewal term shall require the prior approval
        of Owner on a case-by-case basis.

The term "gross rentals" as used in this Exhibit "A" shall mean and refer to the
total of any annual base rent or fixed rent payable by a tenant under any lease,
including leases for expansion space, for an initial lease term or renewal lease
term.

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                                  EXHIBIT B

                               FIRST AMENDMENT TO
                             INSIGNIA/ESG AGREEMENT

                                       9

<PAGE>   10
                                   EXHIBIT B
                     FIRST AMENDMENT TO COMMERCIAL PROPERTY
                              MANAGEMENT AGREEMENT

        This FIRST AMENDMENT TO COMMERCIAL PROPERTY MANAGEMENT AGREEMENT (the
"First Amendment") is made and entered into as of the 6th day of April, 2000,
by and between EBS BUILDING, L.L.C., a Delaware limited liability company
("Owner"), and INSIGNIA/ESG, INC., a Delaware corporation ("Manager").

                                   WITNESSETH:

        WHEREAS, Owner and Manager entered into a certain Commercial Property
Management and Leasing Agreement dated December 31, 1997 (the "Agreement") for
that certain real estate project owned by Owner and improved with buildings and
related parking facilities and common areas (as more fully described therein,
the "Project") commonly known as The Edison Building and located in the City of
St. Louis, Missouri; and

        WHEREAS, Owner has asked to terminate the Agreement effective
immediately with respect to Manager's leasing and marketing rights and
obligations relating to the Project, but otherwise has asked to retain Manager
in connection with the management of the Project, and Manager is agreeable with
such modifications to the relationship of the parties under the Agreement, in
connection with which Manager is willing to waive any sixty (60) day notice
period with respect to cancellation under Section 2.2(d) of the Agreement; and

        WHEREAS, words and phrases having defined meanings in the Agreement
shall have the same respective meanings when used herein, unless otherwise
expressly defined herein; and

        NOW, THEREFORE, in consideration of the foregoing and other
good and valuable consideration, the receipt and sufficiency of which are hereby
acknowledged, the parties hereto agree, effective as of the date hereof, as
follows:

        1. The title of the Agreement and all references to the
Agreement are hereby modified to delete the words "AND LEASING" from the same,
so that the same shall be entitled "Commercial Property Management Agreement."

        2. Section 1.1 of the Agreement is hereby amended in its entirety to
read as follows:

           1.1 Appointment. Owner hereby appoints Manager as the manager of the
Project and Manager hereby accepts such appointment upon the terms, covenants
and conditions set forth herein. Manager agrees to use reasonable efforts and to
cooperate with Owner's leasing agent for the Project, all at no additional cost
to Owner.

<PAGE>   11

        3. No cancellation fee under Section 2.2(d) of the Agreement shall be
payable in connection with this First Amendment and the termination of the
Agreement with respect to Manager's leasing and marketing rights and obligations
relating to the Project.

        4. No cancellation fee under Section 2.3(a) of the Agreement shall be
payable in connection with this First Amendment and the termination of the
Agreement with respect to Manager's leasing and marketing rights and obligations
relating to the Project.

        5. A new Section 2.6 is hereby added to the Agreement to read as
follows:

        2.6 Termination of Leasing Obligations. Manager shall have no leasing
and/or marketing rights and shall have no claims as broker with respect to the
Project from and after the date hereof, except only with respect to those
persons and entities whose lease proposals for a portion of the Project have
been approved by Owner prior to the date hereof ("Manager Contacts"), which
Manager Contract(s), as well as the terms, conditions and commission to be paid
in the event a portion of the Project is leased to said Manager Contact(s), are
detailed on the schedule attached hereto as EXHIBIT "1" and incorporated herein
by reference.

        6. Section 4.2 of the Agreement is hereby deleted in its entirety.

        7. The language of clause (a) of Section 9.2 of the Agreement is hereby
amended by deleting the reference to "Exhibit `D'" and replacing the same with a
reference to "Exhibit `1'".

        8. The first grammatical sentence of Section 9.4 is hereby amended in
its entirety to read as follows:

        "During the term of this Agreement, Owner shall make available to
Manager finished office space (the "Management Office") on a rent-free basis in
an amount not to exceed 3,000 square feet, and sufficient to allow Manager to
provide first class management and construction supervision activities for the
benefit of the Project."

        9. "EXHIBIT D" to the Agreement is hereby deleted in its entirety.

        10. Miscellaneous.

        a.  This First Amendment may be executed in one or more counterparts,
each of which shall be deemed an original and all such counterparts, taken
together, shall constitute but one and the same instrument. Facsimile signatures
on any counterpart shall be effective as an original signature, but the parties
hereto agree to deliver to the other original signatures within thirty (30) days
after the date of this First Amendment.

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        b. Except as expressly amended and modified hereby, all of the terms and
provisions of this Agreement shall remain unchanged and in full force and effect
and are hereby ratified and confirmed.

        c. This First Amendment shall be binding on, and shall inure to the
benefit of, the parties hereto and their respective successors and assigns.

        IN WITNESS WHEREOF, Owner and Manager have respectively signed
this First Amendment to Commercial Property Management Agreement dated and
effective as of the day and year first written above.

OWNER:                                                    MANAGER:

EBS Building, L.L.C.,                                     Insignia/ESG, Inc.
a Delaware limited liability company                      a Delaware corporation

By:   PricewaterhouseCoopers L.L.P.,                      By:/s/  J. John Reis
       its Manager                                           -------------------
                                                             Regional Director

       By:/s/ Keith F. Cooper
          -------------------
             Partner

                                       3

<PAGE>   13

                                   EXHIBIT "1"
                 MANAGER CONTACTS & COMMISSION PAYMENT SCHEDULE

A.      MANAGER ONLY; CO-BROKER NOT INVOLVED:

        DEAL TYPE - NEW LEASE (AS DESIGNATED IN THE CHART ON THE FOLLOWING
        PAGE): Manager shall be paid a commission of 5% of gross rentals to be
        paid by the Manager Contact during years 1 - 5 of the initial term of
        the lease and 3% thereafter.

        DEAL TYPE - RENEWAL (AS DESIGNATED IN THE CHART ON THE FOLLOWING PAGE):
        Manager shall be paid a commission of 2% of gross rentals to be paid by
        the Manager Contact during any renewal term(s) of the lease. This
        provision does not relate to renewals of any new lease entered into with
        a Manager Contact.

B.      MANAGER WITH CO-BROKER INVOLVED:

        DEAL TYPE - NEW LEASE (AS DESIGNATED IN THE CHART ON THE FOLLOWING
        PAGE): Manager shall be paid a commission of 2.5% of gross rentals to be
        paid by the Manager Contact during years 1 - 5 of the initial term of
        the lease and 1.5% thereafter. In addition, the Co-Broker shall be paid
        at the rate of 3.5% for years 1 - 5 and 1.5% thereafter.

        DEAL TYPE - RENEWAL (AS DESIGNATED IN THE CHART ON THE FOLLOWING PAGE):
        Manager shall be paid a commission of 2% of gross rentals to be paid by
        the Manager Contact during any renewal term(s) of the lease. Any fees to
        be paid to a Co-Broker for a renewal term shall require the prior
        approval of Owner on a case-by-case basis. This provision does not
        relate to renewals of any new lease entered into with a Manager Contact.

The term "gross rentals" as used in this Exhibit "1" shall mean and refer to the
total of any annual base rent or fixed rent payable by a Manager Contact under
any lease for the initial lease term or renewal lease term, as the case may be.

A commission will be considered earned if, at any time during the 90-day period
following the date of this First Amendment (the "Initial Period"), Owner (but
not any successor in interest to Owner) enters into a lease for vacant space in
the Project with a Manager Contact as set forth below. In addition, a commission
will be considered earned if at any time during the period commencing ninety-one
(91) days and ending one hundred eighty (180) days following the date of this
First Amendment, Owner (but not any successor in interest to Owner) enters into
a lease for vacant space in the Project with a Manager Contact, provided that
(a) the principal terms of such lease have been previously agreed to in writing
by the Owner and Manager Contact prior to the end of the Initial Period, and (b)
the terms of the lease entered into substantially conform to the terms agreed
upon during the Initial Period.

Commissions shall be paid one-half upon execution of a new lease agreement, and
one-half upon occupancy by the Manager Contact. Commissions for expansions of
space and/or renewal terms shall be paid in full upon execution of the
appropriate documentation.

                                       4

<PAGE>   14

                               ONE FINANCIAL PLAZA
                                MANAGER CONTACTS

<TABLE>
<CAPTION>
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           MANAGER CONTACT                                  ADDRESS                             DEAL
                                                                                                TYPE
------------------------------------------------------------------------------------------------------------
<S>                                     <C>                                               <C>
Inflow                                  1860 Lincoln Street, Dallas, TX                   New
------------------------------------------------------------------------------------------------------------
Anheuser Busch                          701 Market Street, St. Louis, MO                  New
------------------------------------------------------------------------------------------------------------
Evans & Dixon                           100 N. Broadway, St. Louis, MO                    New
------------------------------------------------------------------------------------------------------------
Energizer, Inc.                         Ralston Purina Campus, St. Louis, MO              New
------------------------------------------------------------------------------------------------------------
SW Bell                                 1010 Pine Street, S. Louis, MO                    New
------------------------------------------------------------------------------------------------------------
Habanero Computing Solutions, Inc.      3115 S. Grand, St. Louis, MO                      New
------------------------------------------------------------------------------------------------------------
Sandberg, Phoenix, von Gontard, PC      515 N. Sixth St., St. Louis, MO                   New
------------------------------------------------------------------------------------------------------------
Lewis & Rice                            500 N. Broadway                                   New
------------------------------------------------------------------------------------------------------------
Trilinear Corp                          712 N. Second St., St. Louis, MO                  New
------------------------------------------------------------------------------------------------------------
CCMSI                                   133 S. 11th Street, St. Louis, MO                 New
------------------------------------------------------------------------------------------------------------
White Coleman                           500 Washington Ave., St. Louis, MO                Renewal
------------------------------------------------------------------------------------------------------------
The Witan Company, LLC                  500 Washington Ave., St. Louis, MO                Renewal
------------------------------------------------------------------------------------------------------------
Daniel Henry Insurance                  2350 Market Street, St. Louis, MO                 New
------------------------------------------------------------------------------------------------------------
Secure Networks                         1008 New Hampshire, Kansas City, MO               New
------------------------------------------------------------------------------------------------------------
Goldstein & Price                       100 N. Broadway, St. Louis, MO                    New
------------------------------------------------------------------------------------------------------------
Peavey Barge                            100 N. Broadway, St. Louis, MO                    New
------------------------------------------------------------------------------------------------------------
Michael Fox, Inc.                       500 Washington Ave., St. Louis, MO                Renewal
------------------------------------------------------------------------------------------------------------
Husch & Eppenberger                     100 N. Broadway, St. Louis, MO                    New
------------------------------------------------------------------------------------------------------------
</TABLE>

                                       5<PAGE>   1

PROMISSORY NOTE
---------------

FOR VALUE RECEIVED, the undersigned hereby promises to pay to the order of
Plexus Corp., the sum of Two Hundred Sixty Thousand ($260,000) Dollars, together
with interest thereon at the rate of $250 per month for a maximum of Eight
months (unless extended by mutual agreement) on the unpaid balance.  Said sum
shall be paid in the manner following:  In full within five (5) days after the
undersigned is advised by Plexus Corp.'s counsel (Joseph D. Kaufman) that the
undersigned is not restricted by any law or SEC regulation from selling shares
of Plexus Corp. common stock.

All payments shall be first applied to interest and the balance to principal.

This Note may be prepaid, at any time, in whole or in part, without penalty.

This Note shall at the option of the holder hereof be immediately due and
payable upon failure to make any payment due hereunder, or for breach of any
condition of any security interest, mortgage, pledge agreement or guaranty
granted as collateral security for  this Note, or breach of any condition of
any security agreement or mortgage, if any, having a priority over any security
agreement or mortgage on collateral granted, in whole or in part, as collateral
security for this Note, or upon the filing by any of the undersigned of an
assignment for the benefit of creditors, bankruptcy, or for relief under any
provisions of the Bankruptcy Code, or by suffering an involuntary petition in
bankruptcy or receivership not vacated within thirty days, or if the
undersigned is no longer employed by Plexus Corp.  Additionally, any amount due
holder under said Note may be withheld from any monies due the undersigned from
the holder.

In the event this Note shall be in default, and placed with an attorney for
collection, then the undersigned agrees to pay all reasonable attorney fees and
costs of collection.  Payments not made within 5 days of due date shall be
subject to a late charge of 10% of said payment.  All payments hereunder shall
be made to such address as may from time to time be designated by the holder
hereof.

The undersigned hereby waives demand, presentment and protest and all notices
thereto and further agrees to remain bound, notwithstanding any extension,
modification, waiver, or other indulgence by any holder or upon the discharge
or release of any obligor hereunder or to this Note, or upon the exchange,
substitution, or release of any collateral granted as security for this Note.

The undersigned hereby pledges 4500 shares of Plexus Corp. common stock as
security for this Note, and hereby authorizes any officer of Plexus Corp. to
secure and sell said shares in the event of default by the undersigned, as
outlined in paragraphs 4 and 5 above.

Signed and sealed under penalty of perjury this 13th day of March, 2000.

                                             /s/ Thomas B. Sabol
                                             -----------------------------
                                             Thomas B. Sabol

STATE OF WISCONSIN    )
                      )SS.
COUNTY OF WINNEBAGO   )

     Subscribed and sworn to before me on this 13th day of March, 2000.

                                             Joseph D. Kaufman
                                             ---------------------------------
                                             Notary Public, State of Wisconsin
                                             My commission is permanent

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