A protein related to immunoglobulin light chain detected in mouse myeloma cells.

Several laboratories have recently reported the isolation of the mRNA for immunoglobulin light chain and its translation in cell-free protein synthesizing systems (l-4). In two cases, the products of this cellfree synthesis have been characterized by acrylamide gel electrophoresis (3,4). Milstein et al. (3) have described the properties of a possible precursor of immunoglobulin light chain, synthesized “in vitro” on myeloma or reticulocyte polysomes in response to myeloma mRNA. This putative precursor had a molecular weight somewhat greater than finished light chain, and was not among the products synthesized by myeloma or ascites cell microsomes. Only light chain was detected in the latter systems. Milstein et al. proposed that light chains are synthesized as precursors of somewhat higher molecular weight and are subsequently cleaved to yield the final light chain product. Swan et al. (4) found that mouse myeloma polysomes synthesized several products in response to myeloma mRNA. These included authentic light chain, and proteins with slightly higher and lower molecular weights than light chain. In view of the fact that Swan et al. and Milstein et al. observed the synthesis of several products in response to myeloma mWA “in vitro,” it seemed desirable to examine and characterize the products synthesized by intact myeloma cells. The data presented below indicated that a protein can be detected in cells of the tumor RPC-20 which is similar in structure to the light chain but has a lower molecular weight. MATERIALS AND METHODS