All chemicals were reagent grade and were purchased from Sigma or Fisher unless otherwise stated. Molecular weight standards and all reagents used for SDS-PAGE were from BioRad. The following protease inhibitors were obtained from Sigma: PMSF, benzamidine, Na-tosyl-L-lysine chloromethyl ketone (TLCK), mammalian cell and tissue extracts cocktail 4-(2-aminoethyl)benzenesulfonyl fluoride hydrochloride (AEBSF), aprotinin, bestatin, N-(trans-epoxysuccinyl)-L-leucine 4-guanidinobutylamide (E-64), leupeptin, and pepstatin A. Calcium ionophore A23187 was from Calbiochem. Fluorescent reagents purchased from Molecular Probes included anti-rabbit IgG, anti-mouse IgG, anti-mouse IgM, and peanut agglutinin lectin (PNA) conjugated to Alexa Fluor 555 or 488. Peroxidase-conjugated anti-rabbit IgG (Sigma) and anti-mouse IgG (Jackson ImmunoResearch Laboratories) were used for Western blot analyses.