BBS proteins are necessary for proper function of the regulatory center of the centrosome and spindle pole and for regulation of -tubulin levels. J. Wei¹, H.J. Yen¹, A. Fedler³, Q. Qian², C. Searby¹, M. Andrews¹, D.Y. Nishimura², S. Patil², V.C. Sheffield^1,2. 1) Howard Hughes Medical Institute, Iowa City, IA; 2) Dept of Pediatrics, Univ of Iowa, Iowa City, IA; 3) Univ of Iowa, Iowa City, IA.
   Bardet-biedl syndrome (BBS) is a pleiotropic genetically heterogeneous disorder characterized by obesity, retinopathy, polydactyly, renal malformations, learning disabilities, and hypogenitalism. To date, 11 BBS genes have been identified. Some BBS proteins have been reported to localize to the centrosome. A knockin mouse model of BBS1 (Bbs1^M390R/M390R) and knockout mouse models of BBS2 (Bbs2^-/-), BBS4 (Bbs4^-/-) and BBS6 (Bbs6^-/-) have been developed in our laboratory. These mouse models have features of human BBS. In this study, we utilize cultured mouse renal cells to demonstrate that BBS2 is a microtubule-associated protein and concentrated at the converging sites of microtubules during interphase and at the spindle poles during mitosis. In cultured Bbs2^-/-, Bbs4^-/-, and Bbs1^M390R/M390R renal metaphase cells, we observed that -tubulin is aberrantly associated with spindle microtubules (MTs). Moreover, these mutant renal mitotic cells have aberrant asters that are associated with misalignment of chromosomes. Further investigation of these phenomena revealed that absence of BBS2, or BBS4, as well as the M390R BBS1 mutation, leads to the excessive accumulation of -tubulin. The cause of excessive accumulation of -tubulin was investigated using a MT depolymerization and repolymerization assay. Abnormal regulation of the level of -tubulin was observed in cultured mutant renal cells. In addition, we uncovered the existence of an induced proteolytic pathway that is responsible for the rapid degradation of -tubulin during MT reassembly after the depolymerization of MTs by nocodazole treatment. We propose that BBS proteins are necessary for assembly, maintainence, and disassembly of the regulatory center in the centrosome and spindle poles for rapid regulation of the level of -tubulin. Failure of regulation of level of -tubulin leads to chromosomal misalignment and aneuploidy during mitosis.