We have previously identified murine lung adenoma susceptibility 1 (Las1) as the pulmonary adenoma susceptibility 1 candidate gene. Las1 has two natural alleles, Las1-A/J and Las1-B6. Las1 encodes an 85-kDa protein with uncharacterized biological function. In the present study, we report that Las1 is an unstable protein and the rapid destruction of Las1 depends on the ubiquitin-proteasome pathway. Las1 is a new microtubule-binding protein and Las1 associated with tubulin is not ubiquitinated. We further show that Las1-A/J is a more stable protein than Las1-B6. Las1 is expressed in the G(2) phase of the cell cycle and that ubiquitin-proteasome-mediated Las1 destruction occurs in mitosis. Overexpression of Las1-A/J inhibits normal E10 cell proliferation and induces a defective cytokinesis. The differential degradation of Las1-A/J and Las-B6 has important implications for its intracellular function and may eventually explain Las1-A/J in lung tumorigenesis.