Regulation of microtubule-associated protein 1B (MAP1B) subunit composition

The MAP1B and MAP1A genes each produce an mRNA that encodes a polyprotein. When cleaved, each polyprotein yields a single heavy chain and a single tig ht chain, which become noncovalently associated. In previous work, it was f ound that the MAP1B fight chains and heavy chains exist in a 2:1 ratio. Thr ough use of quantitative immunoblot techniques, this finding was further ex amined in the developing rat brain. MAP1B heavy chain (HC) and light chain (LC1), as well as the light chain of MAP1A (LC2), were prepared in purified form for use as standards and/or immunogens for generation of antibodies f or immunoblotting. Brain homogenates and microtubule-enriched fractions fro m developing rats were assayed for MAP1B HC and LC1 content. Results indica ted that postnatal rat brain homogenates contain LC1 in a 6:1 to 8:1 molar ratio to the MAP1B HC. Purified microtubules also contain LC1 in excess of MAP1B HC, but at a ratio of 2:1. We propose that most of the excess LC1 in homogenates is either insoluble or not bound to microtubules. The findings raise the possibility of a function for the "excess" LC1 that does not requ ire association with MAP1 HC and/or microtubules. Given a synthetic mechani sm that produces MAP1B HC and LC1 in a 1:1 ratio at both transcription and translation steps, we propose that the "excess" LC1 in brain homogenates is a consequence of LC1 having a greater half-life than the MAP1B HC. Consist ently with this hypothesis, a major pool of MAP1B HC is rapidly degraded af ter blocking protein synthesis with cycloheximide, whereas LC1 levels remai n constant even after 24 hr of cycloheximide treatment.. (C) 2000 Wiley-Lis s, Inc.