Evidence for multiple somatic pools of individual axonally transported proteins

The idea that individual axonally transported proteins can exist in several kinetically distinct pools within the cell body was studied using the presumptive neurosecretory low molecular weight (LMW) proteins of Aplysia neurons L11 and R15. Pulse-chase experiments revealed that the loss of labeled LMW proteins from the soma by axonal transport does not follow single exponential kinetics as it should if they are being removed from single pools. Rather, decay of label occurs in at least two phases having half-lives of approximately 1 and 40 h. The LMW proteins are homogeneous by sequential SDS gel electrophoresis and isoelectric focusing, indicating that individual protein species exhibit multiphasic decay kinetics. Two types of evidence imply that the bulk of cellular LMW protein turns over at the slower rate: the LMW pool does not reach constant specific activity at the rapid rate during continuous exposure to labeled precursor, and long-term blockade of axonal transport does not produce an appreciable accumulation of these species in the cell body. These results suggest that some of the newly synthesized LMW protein enters a small somatic pool from which it is rapidly subjected to axonal transport, while the remainder enters a larger pool where it can mix with previously synthesized protein before transport. A cellular mechanism that would yield this behavior is suggested.