Control of antibody biosynthesis

The postulated mechanism whereby secreted proteins are transferred across cellular membranes involves the synthesis of a hydrophobic amino acid sequence at the N-terminus of the protein molecule which triggers the binding of polysomes synthesizing such proteins to the membrane of the endoplasmic reticulum. To determine the presence of such 'precursor' sequences on secreted proteins, it is necessary to translate the appropriate mRNA in a cell-free translation system since the precursor sequence may be cleaved rapidly in vivo. Precursors to several secreted proteins have been identified, including immunoglobulin light chain, but no conclusive evidence for a precursor to immunoglobulin heavy chain has yet been published. The aim of this work was to study the primary translation products of mouse immunoglobulin heavy and light chain mRNAs in a wheat germ cell-free system. The wheat germ system efficiently synthesizes immunoglobulin light chain but more difficulty is experienced in the synthesis of immunoglobulin heavy chain. This phenomenon appears to be the result of ribonuclease activity. The conditions of cell-free translation have therefore been optimized to minimize the effect of mRNA degradation. Immunoglobulin heavy chain was identified by immunoprecipitation and subjected to analysis by peptide mapping and amino acid sequence determination. The presence of a precursor peptide could not be determined by peptide mapping due to inadequate resolution of the peptide fragments. A preliminary amino acid sequence of the N-terminal region of heavy chain synthesized in vitro is, however, indicative of a precursor molecule since the sequence contains several hydrophobic residues and does not contain any of the invariant residues common to the known mouse heavy chain subgroups