Marked Hydrophobicity of the NH2-Terminal Extra Piece of Immunoglobulin Light-Chain Precursors: Possible Physiological Functions of the Extra Piece

mRNAs coding for mouse immunoglobulin light chains direct the cell-free synthesis of precursors in which extra peptide segments precede the NH2-termini of the mature proteins. The abundance (18-30%) of leucine residues in the extra piece indicates that it is quite hydrophobic [Schechter and Burstein (1976) Biochem. Biophys, Res. Commun. 68, 489]. Accordingly, we have determined the positions of all hydrophobic residues by sequencing two k-type light (L)-chain precursors that were labeled with: [3H]Ala, [3H]Val, [3H]Leu, [3H]Ile, [3H]Thr, [3H]Pro, [3H]Phe, [3H]Tyr, [3H]Trp, [35S]Met, and [35S]Cys. The partial sequences (and sizes) of the extra pieces obtained are: in MOPC-321 precursor, Met-X-Thr-X-Thr-Leu-Leu-Leu-Trp-Val-Leu-Leu-Leu-Trp-Val-Pro-X-X-Thr-X-(20 residues; X is unknown); in MOPC-41 precursor, Met-X-Met-X-Ala-Pro-Ala-X-Ile-Phe-X-Phe-Leu-Leu-Leu-Leu-Phe-Pro-X-Thr-X-Cys- (22 residues). Despite the fact that these extra pieces differ extensively in sequence (68%), both of them are highly enriched with hydrophobic residues (75% in MOPC-321, 73% in MOPC-41). This marked hydrophobicity suggests that the extra piece favors interaction of the precursor with cell membranes, in a manner similar to the function of the "hydrophobic domain" of membrane-bound proteins (e.g., glycophorin). We propse that the hydrophobic extra piece directs most precursor molecules to the endoplasmic reticulum, where they are cleaved to yield mature L chain destined for scretion; a few precursor molecules escape cleavage and are embedded in the cell surface to serve as the antigen-recognizing receptor. The probability that the Leu-Leu-Leu-Trp-Val sequence occurs by change is 1.6 X 10(-8). Therefore, the data provide evidnece for duplication of a short DNA segment in the structural gene coding for the MOPC-321 precurosr. Duplication with inversion is also indicated from inverted repetition of the Phe-Lue-Leu sequence in the extra piece of the MPOC-41 precursor.