THE FUNCTIONAL UNITS OF A PEPTOSTREPTOCOCCAL PROTEIN-L

Protein L is a cell-surface protein from Peptostreptococcus which inte racts with immunoglobulin kappa light chains. A gene from Peptostrepto coccus strain 3316 coding for protein L and fragments thereof were exp ressed in Escherichia coli. The peptides were examined for binding to immunoglobulin and serum albumin. The four C units were shown to be re sponsible for binding to immunoglobulin and the four D units for bindi ng to albumin. This protein L molecule therefore binds to albumin at a site separate from that involved in binding to immunoglobulin. The al bumin-binding units have high amino acid sequence identity with the al bumin-binding units of streptococcal cell-surface proteins. The gene c ontains three sites available for Internal initiation of translation r esulting in three active proteins. The protein L molecule presented in this report was compared with a previously reported protein from Pept ostreptococcus strain 312. The two proteins differ in several respects , including size and the number and types of repeat units.