Axonal transport of microtubule-associated protein 1B (MAP1B) in the sciatic nerve of adult rat: Distinct transport rates of different isoforms

Cytoskeletal proteins are axonally transported with slow components a and b (SCa and SCb). In peripheral nerves, the transport velocity of SCa, which includes neurofilaments and tubulin, is 1-2 mm/d, whereas SCb, which includ es actin, tubulin, and numerous soluble proteins, moves as a heterogeneous wave at 2-4 mm/d. We have shown that two isoforms of microtubule-associated protein 1B (MAP1B), which can be separated on SDS polyacrylamide gels on t he basis of differences in their phosphorylation states (band I and band II ), were transported at two different rates. All of band I MAP1B moved as a coherent wave at a velocity of 7-9 mm/d, distinct from slow axonal transpor t components SCa and SCb. Several other proteins were detected within the c omponent that moved at the velocity of 7-9 mm/d, including the leading wave of tubulin and actin. The properties of this component define a distinct f raction of the slow axonal transport that we suggest to term slow component c (SCc). The relatively fast transport of the phosphorylated MAP1B isoform at 7-9 mm/d may account for the high concentration of phosphorylated MAP1B in the distal end of growing axons. In contrast to band I MAP1B, the trans port profile of band II was complex and contained components moving with SC a and SCb and a leading edge at SCc. Thus, MAP1B isoforms in different phos phorylation states move with distinct components of slow axonal transport, possibly because of differences in their abilities to associate with other proteins.