With the use of the
monoclonal antibody BA-1, we have identified and characterized a unique molecular complex on the surface of human B lymphocytes. Despite the marked
pronase sensitivity of the
BA-1 antigen/
epitope on the cell surface, repeated attempts to surface or biosynthetically radiolabel the
antigen by using standard
protein labeling techniques were unsuccessful. The
antigen was identified by immunoprecipitation and
sodium dodecyl sulfate-
polyacrylamide gel electrophoresis after radiolabeling
carbohydrate residues. Surface labeling of galactosyl residues by the
neuraminidase/
galactose oxidase/3HNaBH4 method revealed a three chain, nondisulfide-linked
glycoprotein complex of 45, 55, and 65 kilodaltons (kDa), which we have designated gp45/55/65. Under nonreducing gel conditions there were shifts in the mobility of the 55- and 65-kDa components, consistent with the presence of intrachain
disulfide linkages. Surface labeling of
sialic acid residues by the
sodium periodate/3HNaBH4 method revealed the presence of
sialic acid residues on the 45- and 55-kDa components. Two-dimensional gel analysis indicated that the 55-kDa component had an acidic pI of approximately 4.0 and considerable charge heterogeneity, most likely attributable to
sialic acid. In contrast, the 45-kDa component had a neutral pI of approximately 7.5. Despite the difficulty in radiolabeling
amino acids in gp45/55/65, digestion with V8
protease and
pronase clearly demonstrated the presence of
protein cores in all three components. Consonant with prior serologic studies with the use of
BA-1, we could identify gp45/55/65 on B lymphocytes, B cell precursor acute lymphoblastic
leukemias, neutrophils, and eosinophils. This molecular complex has, to our knowledge, not been heretofore described on human B cells and is quite novel in its structure and labeling properties.