Plasma cell immunoglobulin M molecules. Their biosynthesis, assembly, and intracellular transport.

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Abstract
Immunoglobulin (Ig)M-screting murine plasmablasts were used to explore cytologic site(s) of successive modifications of polypeptide H and L chains (steps of glycosylation, chain assembly and polymerization) which occur during intracellular transport (ICT) and the interrelationships between these events. A combination of pulse-chase biosynthetic labeling protocols (using amino acids and sugars), subcellular fractionation and electron microscope autoradiography was used in conjunction with inhibitors of glycosylation and agents (carboxyl cyanide m-chlorophenyl hydrazone [CCCP] and monensin) which blocking exit from the rough endoplasmic reticulum (RER) or Golgi cisternae. The data are consistent with the following conclusions. Sugar addition and modification occur in 3 main steps: en bloc addition of core sugars to nascent H chains, partial trimming of these oligosaccharide chains in the RER and quasiconcerted addition of terminal sugars (galactose, fucose and sialic acid) in a very distal compartment between monensin-sensitive Golgi cisternae and the cell surface. H and L chain assembly occurs between nascent H chains and a pool of free L chains present in the RER, followed by interchain disulfide bonding and rapid assembly of monomers into J chain-containing pentamers in the RER. Small amounts of various, apparently non-obligatory intermediates in polymerization, are also formed. Carbohydrate addition is not required for chain assembly, polymerization and secretion since completely unglycosylated chains (synthesized in the presence of deoxyglucose or tunicamycin) undergo polymerization and are secreted (although at a reduced rate). Surface 8s IgM molecules do not represent a step in the IgM secretory pathway.

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