A structural feature of human immunoglobulin light chains. Two compact domains connected by a small switch region.

A lambda immunoglobulin light chain from a myeloma patient, normal light chains, and fragments one-half the size of these chains have been examined by chemical and physical methods. Fragments produced from the X chain with trypsin, pepsin, and papain, and free urinary fragments of this chain were isolated. Characterization of the various fragments of the X chain by amino acid analyses, peptide mapping, NH2-terminal sequence determinations, gel chromatography, and ultracentrifugations suggested that the fragments consist of whole variable or constant halves, and that they extend one to five residues into the adjacent half. Thus, the region at the mid-point of the chain which is particularly susceptible to proteolysis apparently consists of a polypeptide stretch of less than ten residues. This region seems to be of similar size in normal light chains, since fragments of such chains had very similar Stokes’ radii and molecular weights as halves of the X chain. The urinary fragments of the X chain were identical or similar to fragments formed by proteolysis. These fragments probably are of catabolic origin. The halves of the light chains had significantly lower frictional ratios than the intact chains, which suggests that whole light chains are more elongated than the halves or contain trapped water in the contact region between the halves (or both).