C-Terminal Half of Immunoglobulin λ Chains

Abstract

Two types of chains (ϰ and λ chains) comprise most, if not all, of the heterogeneous population of light chains of pooled normal human gamma-globulins^1–3. The C-terminal half of ϰ chains appears to be identical in proteins derived from different clones (myelomas)^4–6 except for a single residue which is correlated with a genetic marker^6–7. Invariance of the C-terminal half of mouse ϰ chains has also been reported⁸. A similar situation is found in λ chains. Following a previous report containing the sequence of some sections of the C-terminal half^9–10, we propose in this communication the sequence of the C-terminal half of a λ Bence Jones protein (X) (Fig. 1). A second λ Bence Jones protein (Mz) has been compared with the first by isolating tryptic peptides and determining their amino-acid compositions. In a comparison of the two λ chains the only difference detected in the C-terminal half was the substitution of a serine for an arginine molecule at residue 107 (we have kept a unified numbering system for the sequences shown in Fig. 1 in order to simplify the comparison). The position of this substitution (residue 107) is identical to the last position of the N-terminal half of ϰ chains containing sequence variations⁴. It seems, therefore, that the variable stretches of the sequences of both types of light chains may end in an identical position when the homologous sequences are compared. This is compatible with the hypothesis that the variability of immunoglobulin chains is introduced by a hypermutation process having a well defined starting signal common to all the chains¹⁰, although other explanations are not excluded.