T HE cytoplasmic domains of a number of transmembrane membrane proteins are thought to be important in the membrane biology of those proteins (3, 13, (16) (17) (18) 31) . Plasma membrane proteins with truncated cytoplasmic domains may not be correctly sorted or internalized and may lose signaling functions (22) (23) (24) 26) . Surprisingly, the translational diffusion coefficient (D,.t)' of proteins with such truncations is not found to differ from that of full-length molecules. Dj., of the EGF receptor (16) , class I major histocompatibility complex (MHC) antigen (10) , or G protein of vesicular stomatitis virus (25) is not changed when the cytoplasmic domains of these molecules are truncated by directed mutation of their genes. At the extreme, this involves a reduction from 542 to 9 residues in the EGF receptor (16) . These studies imply that the amino acids removed are not associated with cytoplasmic proteins in ways that constrain their translational diffusion. There is an apparent contradiction between the results on the cell physiology of truncated molecules and the measurements of their translational diffusion. Results of endocytosis experiments indicate that the molecules are coupled to the cell cytoskeleton and that this coupling is lost with truncation since internalization is affected by truncation. On the other hand, the results of translational diffusion experiments suggest that even wild-type, 1. Abbreviations used in this paper: D0~t, translational diffusion coefficient; FPR, fluorescence photobleachin 8 and recovery; MCF, mean channel fluorescence; MHC, major histocompatibility complex; PKC, protein kinase C. full-length molecules are not directly linked to the cytoskeleton since truncation of their cytoplasmic domains has no effect on D,.,. This contradiction may arise largely because translational diffusion and endocytosis of truncated molecules have been studied in different cell types. D~ of the truncated class I MHC mutants has been measured in L cells, in which class I molecules are not rapidly internalized. The effect of truncation on endocytosis of the same mutants is readily shown when they are expressed in T lymphoma cells, a cell type in which there is continuous cycling of class I MHC antigens between the surface and cytoplasm (4) . Similarly, D,,, of truncated EGF receptor was measured in COS cells, while effects of truncation on endocytosis of EGF receptor were shown in 3T3 cells (2) . Here we present the first results in which D,a, of truncated transmembrane molecules is measured in cells in which the wild-type molecules are functional. Truncation of the cytoplasmic domain affects D,a, in these cells; in the extreme truncation mutant, the D,,, is an order of magnitude larger than that of the wild-type molecule.
Class II MHC molecules, Ia antigens in the mouse, are heterodimeric transmembrane proteins expressed on B lymphocytes, macrophages, and other antigen-presenting cells. There are two isotypes of Ia antigens in the mouse, I-A and I-E; each consists of a 34-36-kD ct chain and a 27-29-kD/~ chain. Each chain is composed of two external domains (ot~/ot2 or/3,/~2) and a connecting peptide that joins the ex-ternal domains to the transmembmne and cytoplasmic domains. There are two asparagine-linked oligosaccharide units on each c+ chain and one on the B chain.
One function of Ia molecules is to present foreign peptide ~wt/[~wt antigens to T lymphocytes. Antigen presentation in B lymphocytes involves facilitated internalization of native antigen by antigen-specific surface Ig (signaling aspects reviewed in reference 6) followed by processing of the foreign antigen by denaturation and/or fragmentation. Ia molecules also function as receptors that influence the differentiative or proliferative events in B lymphocytes (6, 7) . Potentially, Ia molecules are coupled to the molecules of the signal-transducing cascade through the highly conserved amino acids of the transmembrane domain or cytoplasmic domain (12) . Ia molecules and surface Ig (which also provides signals to B cells) are similar in their ability to associate with insoluble cytoskeletal elements after receptor cross-linking and to remain associated with the cytoskeleton during solubilization of the cells (30) . These cytoskeletal associations may be important for the signaling function which Ia and Ig molecules provide.
B cell lymphomas transfected with class II genes have also been used to study one aspect of B lymphocyte signaling, the translocation of protein kinase C (PKC) from the cytoplasm to the nucleus, that is induced by perturbation of membrane Ia molecules by Ia-specific rnAbs. This binding of surface Ia by mAbs presumably mimics the interaction of T cell receptors with peptide antigen-bearing Ia molecules. I-A k molecules consisting of truncated/~ chains paired with either fulllength ot chains (ctwt//~CT12) or with truncated ot chains that had no cytoplasmic domain amino acids (otCT12/flCT12) had altered PKC signaling phenotypes compared with wild-type I-A k molecules. Further, I-A k molecules that lacked cytoplasmic domains of both a and/3 chains (aCT12//3CT18) were unable to signal at all as measured by the translocation of PKC from the cytoplasm to the nucleus (28) .
These results collectively suggest that the cytoplasmic domain of I-A k is functionally coupled to a signal transduction pathway and to the cytoskeleton in B lymphocytes. Given this coupling, it might be expected that deletions of all or part of Cells the cytoplasmic domain would affect the D,= of the mutant Ia molecules. Using fluorescence photobleaching and recovery (FPR) measurements, we have determined the D~,, of wild-type I-A k molecules and of mutant I-A k molecules lacking all or part of the cytoplasmic domain of one or both chains. Here we show that the D~= of I-A k molecule is a function of the length of its cytoplasmic domains.
The pSV2-Neo construct (27) was a gift from Dr. Ed Palmer (National Jewish Center for Immunology and Respiratory Medicine, Denver, CO). The origin and generation of the a and fl I-A k genomic DNA constructs have been described (28) . In brief, site-directed mutagenesis in vitro was used to introduce double-stop codons that v~uld result in truncated a or fl chains lacking 6 and 12 or 12 and 18 amino acids, respectively. Uamb+nne I RHRsQK Transfectants were generated by introducing pSV2-Neo DNA and ~ and genomic DNA into the Ia-negative B cell iymphoma M12.C3 by electroporation. The transfecled cells were selected and maintained using (3418 (neomycin) at a final concentration of 300 t~g/ml. Transfectants expressing memk k brahe I-A were chosen based on flow cytometry using anti-l-A -specific mAb (Fig. 2) .
mAbs were purified from tissue culture supernatants by affinity chromatogk raphy on protein A-Sepharose. The anti-l-A mAb, 39], is specific for the c~ chain (21) . The anti-I-A k mAb, 10-3.6, is specific for the fl chain (19) . The anti-L d mAb, 28-14-8, was used as a control to assess for nonspecific effect on plasma membrane protein Dl,, values (20),2-8 mg of mAb was digested with 1:100 (wt/wt) papain (Sigma Chemical Co., St. Louis, MO) at pH 5.2 for 10 min. The digest was fractionated on Sephadex G-100, and the first included peak was refractionated on protein A-sepharose to remove traces of IgG. The purified Fab was concentrated to 5-10 mg/mi protein, dialyzed into 0.2 M Na2CO3 buffer, pH 9.5, and reacted with 15 #g FITC/ mg protein at 0oc for I h. The reaction mixture was purified by passage c~er Sephadex G-25. All conjugates had 3-4 tool fluoresccin coupled/mol Fab.
In an FPR experiment, diffusion in the plane of the membrane, translational diffusion, is measured in terms of recovery of fluorescence after partly bleaching a small area on an otherwise uniformly labeled cell surface (8, I  I I II IIII  I  I  I I 14). The recovery curve contains information about the Dlat of the mAbbound molecules and about the fraction, R, of the population of mAb-bound molecules that are free to diffuse, on a time scale of minutes, in the experiment. Dl,t of a given molecule is a function of the radius of the diffusing species. Hence, monovalent labels-i.e., Fab fragments (which do not cross-link bound molecules)-give more reliable estimates of Dlat than do larger and bivalent IgG labels. The mobile fraction, R, is influenced by the intensity and specificity of binding of the Fab, as well as by the biology of the ceils. Calculations and control experiments indicate that the measuring and bleaching beams do not cause detectable cross-linking of, or damage to, the low concentrations of molecules of interest in an FPR experiment (9) .
In our experiments, cells were labeled with FITC-Fab for 15-30 min and washed four times in Hepes-buffered Hanks' balanced salt solution. Most measurements were made on cells in suspension and at 20"C. Our FPR machine has been described elsewhere (8) . In one experiment, there was no difference between the Dl=t measured on cells in suspension and Dtat measured on cells adherent to coverslips (data not shown). There was also no difference between Dlat at 20°C and Dlat at 30°C (data not shown).
In experiments with MI2.C3-10-D2, the transfectant that expresses the lowest level of I-A k, recovery of fluorescence was reduced if cells were incubated with goat anti-mouse IgG before measurement (data not shown). This demonstrates that the fluorescence measured is from mAb bound to surface I-A k and is not cell autofluorescence.
The values of Dlat typically are not a normal distribution. Accordingly, geometric mean Dlat were calculated from normally distributed laD. Signifieances of differences between mean InDRit were compared in Student's t test. p < 0.05 was considered significant.
Transfectants expressing various combinations of truncated t~ or B chains were generated, and the translational diffusion of the class II molecules was measured by FPR. Fig. 2 shows representative fluorescence profiles of the transfectants expressing either wild-type or mutant I-A ~ molecules. The level of I-A k expressed at the cell surface differs by no more than twofold between these transfectants, with the exception of the otCT12//~CT18 truncation phenotype transfectant. It can be seen from Table I that removal of the carboxy-terminal 12 (ofa total of 18) amino acids of the/3 chain does not affect D~,~. Wild-type I-A k and o~wt//~CTl2 I-A ~ molecules have the same Dt~. This contrasts with the effect measured after the loss of as few as six amino acids from the tx chain. D~, of I-A k molecules of the ctCT6 or the t~CT12 phenotypes paired with a wild-type B chain is significantly higher than D~, of wild-type I-A k molecules. It seems that the ot chain of the I-A k molecule constrains its diffusion in the plasma membrane. This conclusion is supported by the results with the transfectant M12.C3-5-B2 with the I-A ~ truncation phenotype otCT12//~CTI2. D~, of otCTI2//~CTI2 I-A ~ molecules does not differ from that of transfectants of the otCT6/Bwt and o~CT12/Bwt phenotypes.
It appears from these results that 12 of the 18 amino acids of the cytoplasmic domain of the/~ chain have little influence on the Dn~ of I-A k molecules and that the tx chain is primarily responsible for constraining the translational diffusion of I-A k. To assess the contribution of the six membranepromixal amino acids of the cytoplasmic domain of the/3 chain, we generated transfectants expressing I- For ease of comparison, the normalized fold increase of D~,, for individual I-A k phenotypes is shown in Fig. 3 . The order of increasing D~t is otwt/Bwt = txwt/BCT12 < txCT6/ /~wt = aCT12//~wt = oA2T12/BCTI2 < CT12/CTI8. The fold increase from wild type describing this order is 1 = 1.5 < 3.1 = 3.0 = 3.5 < 9.9. Statistical significance for various comparisons of the D~,t is summarized in the legend for Table I. All comparisons between D~, of I-A k on individual Table I. transfectants are only made within an experiment. Dj,, were independent of the differences between experiments in the mobile fractions (see below).
The mobile fraction of wild-type and mutant I-A k antigens was constant within an experiment with one exception. The mobile fraction of the tailless I-A k molecule (t~CT12//~CT18) was significantly (p < 0.05) higher than that of the wild-type I-A k in both experiments 3 and 4.
To ensure that D~,, measured for the transfectant M12.C3-10-D2 (otCTI2//3CT18) was in fact due to the truncation phenotype of the I-A k molecule and not the result of the low concentration of I-A k protein, we used cell sorting to enrich for M12.C3-10-D2 cells that expressed the highest level of the t~CT12/BCT18 I-A k molecule. A fluoresceinated ct chainspecific mAb was used to label cells that were sorted on a cell sorter (Epics 751; Coulter Electronics Inc., Hialeah, FL), and the 10% of the cells expressing the highest levels of trun-cated I-A k were selected. The resulting sorted population is shown in Fig. 2 D. A normally distributed population of ¢xCT12//3CT18 I-A k expressing cells can be seen using either directly conjugated Fab of mAb 39J (<x chain specific) or 10-3.6 (13 chain specific) for staining. This sorted population, M12.C3-10-D2.Hi, was then used for FPR measurements to determine the D~,, of the mutant I-A k. Fig. 4 and Table I show the data of the D~,, for M12.C3-F6 and MI2.C3-10-D2.Hi. The D,,, for the otCT12//3CT18 I-A k molecules of the sorted population is still •10-fold higher than wild-type I-A k molecules, whether an c~ or/3 chain-specific mAb is used. These results demonstrate that even for the a, CTI2//3CT18 truncation, using an unsorted population in which only a small number of cells clearly express the truncated I-A k molecule above background levels produces the same D~,, as the use of a sorted population in which the expression of truncated I-A k is nearly baseline resolved from the negative control.
The differences we measured in D~,, for I-A k molecules in transfectants expressing truncated I-A k molecules could be due to clonal variation in constraints to diffusion of plasma membrane molecules in the individual transfectants. To control for this, we measured translational diffusion of constitutively expressed class I antigens of the transfectants (Table  II) . D~,, for the L d molecule are the same regardless of which transfectant is measured with the exception of the M12.C3-5-B2 (otCT12/BCTI2) cell line. Ln Dj,t for L a in this clone is smaller than the others. This difference does not correlate with Dma, for truncated I-A ~ molecules (compare Fig. 3 with Table II ) and may be due to the high standard deviation of D,,, for the otCT12//3CTI2 transfectant. Hence, the differences in the D,,, of 1-,4, k molecules in the various transfectants arise from truncation of the ot or/3 chains and not from variations of the dynamics of the plasma membranes of individual transfectants.
The translational diffusion of I-A k antigens expressed in B lymphoblasts is affected by truncation of the cytoplasmic domains of these antigens. Loss of 6 of 12 cytoplasmic residues of the ~ chain results in an approximately threefold increase * Calculated from InD whose mean and standard deviation are shown in parenthesis. * Student's t test was used to compare D~ of L J molecules expressed on otwt/ tiwt with D~ for L J expressed on cells with various I-A ~ truncation phenotypes. None of the comparisons were significant at the p = 0.05 level with the exception of aCTI2/BCT12.
in D~, relative to wild-type I-A k antigens. Deletion of the remaining six residues of the et chain cytoplasmic domain has no further effect on D~,,. This is true whether the mutant ot chain is paired with a wild-type/3 chain or whether it is paired with a/3 chain lacking 12 of 18 of the amino acids of its cytoplasmic domain. Thus, the length of the/3 chain has seemingly little effect on D~,,. However, complete deletion of the/3 chain tail results in a further approximately threefold increase in Dt~, when the mutant chain is paired with an chain also completely deleted of its cytoplasmic domain. This D~,~ is still smaller by about a factor of five than might be expected for a molecule of the size of I-A k diffusing in a lipid bilayer. The bulky oligosaccharide units of I-A k may impede their diffusion, as has been found for class I MHC antigens (29) .
The mobile fraction of wild-type I-A k was lower in experiments 3 and 4 than in experiments 1 and 2. Some of this variation must be due to biological rather than technical factors (such as microscope alignment or nonspecific binding of label) since in experiment 4 two different FITC-Fab labels, one for the oz and one for the/3 chain, both report low mobile fractions of wild-type I-A k and higher mobile fractions of tailless I-A k (ozCT12//3CTI8). The mobile fraction of surface molecules on cells from dense cultures is often lower than the mobile fraction of the same molecules on cells from tess dense cultures (3, 29) and it also is reduced with long times of labeling (3) . The cultures used in these experiments appeared to be at about the same cell density and were labeled to the same extent. Hence, it is not clear if any of the cited factors are involved. It is evident that the mobile fractions of tailless I-A ~ molecules are significantly higher than that of wild-type or etwt//3CT12 truncates (experiment 3). The complete deletion of the tails appears to uncouple the mutant molecules from the (unknown) structures that immobilize surface molecules (cf. 15).
Our previous results show that the associations of I-A k with molecules that may be involved with the Ia-mediated signaling cascade are controlled by the/3 chain (18) . This hypothesis is supported by the fact that I-A k molecules conraining a truncated ~ chain paired with a wild-type ot chain still signal (albeit with an altered phenotype) but have a wildtype D,,,. Further, the ability to mediate PKC translocation through I-A ~ can be abolished by the complete removal of all ~] chain cytoplasmic domain amino acids. This negative PKC signaling phenotype is correlated with the large increase in D~,, of I-A k molecules of the t~CT12//3CT18 phenotype, almost 10 times higher than that of full-length I-A k molecules and 3 times higher than that of I-A k molecules of the o~CTl2//~wt phenotype.
We suggest that the inability of the c~CTI2//~CT18 I-A k phenotype to signal is due to the fact that an I-A k molecule containing a ~ chain that is missing all its cytoplasmic domain amino acids is unable to associate with the molecules of the Ia-mediated signaling cascade (5, 6, 28) . These signaling cascade molecules may be proteins responsible for the proximal events in Ia-mediated signaling: those causing an increase in cAMP and/or translocation of PKC to the nucleus.
The truncation of the a chain alone does not significantly affect signaling but does increase lateral diffusion. The fact that in these molecules PKC signaling is still intact indicates that D~, may increase threefold without the I-A k molecule uncoupling from the signal transduction pathway. There are a number of possible explanations for this observation. One possibility is that the interaction between the Ia molecule and the molecule(s) of the signaling pathway is transient and is mediated almost exclusively by the/3 chain. Thus, the requisite interaction can occur with normal kinetics in spite of the fact that the Ia molecule is diffusing threefold more rapidly. Additionally, truncation of the /3 chain in aCT12//3CTI8 I-A k molecules removes the six plasma membrane-proximal amino acids of the ~ chain that are required for constitutive binding of the proteins of the Ia-mediated signaling cascade. This binding contributes to the wild-type D~a,, and therefore the loss of the binding results in an increase in Dja~ as well as in inability to translocate PKC. In our view, lateral diffusion of I-A k reflects two independent interactions: one in which the a chain interacts with cytoskeletal elements that are not necessary for PKC translocation and one in which the /3 chain interacts with molecules that are required for PKC translocation.
Preliminary results (Wade, W. E, E. E Rosloniec, and J. H. Freed, unpublished observations) indicate that antigen presentation by I-A k molecules is affected by the length of the et chain cytoplasmic domain. A published report also indicates that antigen presentation is affected by truncation of the ct chain's cytoplasmic domain (13) . While the mechanism underlying these observations is unknown, it is tempting to speculate that there is a causal relationship between the increased D~, for I-A ~ molecules with truncated et chains and their reduced ability to present antigen. Although it is possible that the increased mobility of the I-A k molecules with truncated c~ chains could be directly responsible for this effect, we currently favor the alternate hypothesis that the antigen presentation defect derives from the altered transport, endocytosis, and/or reexpression of I-A k molecules with truncated t~ chains.
The D~a, we measured and the percent mobile fraction of wild-type I-A k is quite close to that reported for Ia in another cell line, A20.3 (Bariasas, B. G., D. A. Roess, H. M. Grey, and T. M. Jovin, unpublished observation). Another reported value for D,a, of I-A k is far larger, 4 x 10 -9 cm2/s (13) . The reason for this difference is not clear, but we emphasize that cell type and experimental protocols are important variables affecting measured D,~t and mobile fraction.
The increase in Dt~t as a result of truncating the cytoplasmic domains of ct or/3 chains is different from that reported by one of us for class I molecules, where truncation of the cytoplasmic domain did not alter the D~,t value compared with that of the full-length molecule (10) . The observed differences between the behavior of truncated class I and class II molecules may be explained by the fact that the class I molecule has a single membrane domain whereas the class II molecule has two. An alternate, and not mutually exclusive, explanation may be that class I molecules may not have a signaling function in the L cells in which they were studied and thus are not associated with cytoskeletal elements (4) . Additional studies will be required to provide a detailed explanation for the observed differences between the D,~, of truncated class I and class II molecules. Ultimately, however, studies of their diffusion should provide fundamental insights into how the physicochemical properties of the class II molecules dictate their function as signal-transducing membrane receptors.