IDENTIFICATION OF ALBUMIN-BINDING PROTEINS OF THYMOCYTE PLASMALEMMA

In the present work we examined whether the interaction between albumi n molecules and thymocytes involves albumin-binding proteins (ABP). Tw o plasmalemma-rich fractions obtained by differential centrifugation f rom rat thymus lymphocytes were characterized biochemically and morpho logically. These fractions were examined by ligand-blotting and ligand affinity chromatography techniques. Plasmalemma proteins separated by SDS-PAGE were electrotransferred onto nitrocellulose membranes and in cubated with I-125-albumin, in the presence or absence of excess nativ e albumin. The autoradiogram revealed specific binding to two sets of polypeptides of 16-18 and 29-31 kDa, which could be blocked by native albumin. To elucidate whether albumin-binding proteins are exposed on the cell surface, intact lymphocytes were surface radioiodinated and m embrane fractions prepared from them were subjected to affinity chroma tography on albumin-agarose beads. The proteins thus purified had, lik e ABP, M(r) of 16 and 31. These data indicate that ABP (i) are compone nts of thymocyte plasma membrane, (ii) have apparent molecular mass of 16-18 and 29-31 kDa, and (iii) are exposed on the outer membrane surf ace.