Transplantable immunoglobulin-secreting tumors in rats. VI. N-terminal sequence variability in LOU/C/Wsl rat monoclonal heavy chains.

Most of the spontaneous ileocecal immunocytomas of LOU/C/Wsl strain rats secrete large amounts of monoclonal immunoglobulins. Amino acid sequence studies were undertaken on the heavy chains of 13 of these proteins and on pooled heavy chains from LOU rats to define the structural variability in the amino terminal region of the rat heavy chain. Characteristic VHIII sequences were found in both the myeloma proteins and the heavy chains from the pool. The data confirm previous conclusions concerning the rat VHIII subgroup that had been derived from studies of pooled heavy chains from Sprague-Dawley rats. In particular phylogenetically associated residues, identical to those previously determined in the Sprague-Dawley pool, were found in the LOU/C/Wsl rat myeloma proteins. There were two unexpected findings: 1) hypervariability at position 16 of the rat VHIII myelomas as contrasted with more than 90% glycine in the pool, and 2) a striking homology of one rat myeloma heavy chain with the variable region of the MOPC 315 mouse alpha-chain. The findings show that the rat myeloma system provides useful proteins for sequence analysis but indicate that some sequence pattern differences can be detected between the myeloma proteins and pooled immunoglobulins from this species.