Glutamine as a precursor to N-terminal pyrrolid-2-one-5-carboxylic acid in mouse immunoglobulin λ-type light chains. Amino acid-sequence variability at the N-terminal extra piece of λ-type light-chain precursors

The mRNA molecules coding for three mouse immunoglobulin λ-type light (L) chains (MOPC-104E λ₁, RPC-20 λ₁, MOPC-315 λ₂) programme the cell-free synthesis of precursors larger than the mature proteins. Radioactive amino acid-sequence analyses of each of the three precursors labelled with [³H]alanine, [³H]serine, [³H]glutamine, [³H]glutamic acid and [³H]threonine showed that an extra piece, at least 18 residues long, is linked to the N-terminus of the mature L-chains. The N-terminal extra-peptide segment may be 19 residues long, since analyses of precursors labelled with [³⁵S]methionine indicated an additional N-terminal methionine residue which was recovered in low yields. Presumably this is the initiator methionine, which is known to be short lived in eukaryotes. The mature forms of MOPC-104E, RPC-20 and MOPC-315 λ L-chains are blocked at the N-termini by pyrrolid-2-one-5-carboxylic acid (pyroglutamic acid). Sequence analyses of precursors labelled with [³H]glutamine and [³H]glutamic acid showed incorporation only of glutamine in a position that matches with the position of pyrrolid-2-one-5-carboxylic acid in the mature forms of all three precursors, and incorporation of glutamic acid in other positions. The data showed the absence of glutamine–glutamic acid interconversion, since the radioactive peaks obtained from either³H-labelled amino acid were discrete, and free from cross-contamination. These results prove that glutamine is the precursor amino acid of pyrrolid-2-one-5-carboxylic acid at the N-termini of the mature MOPC-104E λ₁, RPC-20 λ₁ and MOPC-315 λ₂ L-chains. Thus the formation of pyrrolid-2-one-5-carboxylic acid by cyclization of glutamine is a post-translational event which occurs after, or concomitant with, cleavage of the extra piece from the precursor to yield the mature L-chain. The variable (V) regions (110 amino acid residues) of mouse λ L-chains are quite similar: when compared with that of MOPC-104E λ₁ chain, the V-region of RPC-20 λ₁ chain differs in one residue, and the V-region of MOPC-315 λ₂ chain differs in 11 residues. The partial sequence data show that the N-terminal extra pieces of the two λ₁ L-chain precursors have, so far, identical partial sequences; the extra piece of the λ₂ L-chain precursor differs from these in at least three out of 19 positions.