A germline-specific role for unconventional components of the ?-tubulin complex in Caenorhabditis elegans

The ?-tubulin complex (?TuC) is a widely conserved microtubule nucleator, but some of its components, namely GCP4, GCP5 and GCP6 (also known as TUBGCP4, TUBGCP5 and TUBGCP6, respectively), have not been detected in Caenorhabditis elegans. Here, we identified two ?TuC-associated proteins in C. elegans, GTAP-1 and GTAP-2, for which apparent orthologs were detected only in the genus Caenorhabditis. GTAP-1 and GTAP-2 were found to localize at centrosomes and the plasma membrane of the germline, and their centrosomal localization was interdependent. In early C. elegans embryos, whereas the conserved ?TuC component MZT1 (also known as MOZART1 and MZT1) was essential for the localization of centrosomal ?-tubulin, depletion of GTAP-1 and/or GTAP-2 caused up to 50% reduction of centrosomal ?-tubulin and precocious disassembly of spindle poles during mitotic telophase. In the adult germline, GTAP-1 and GTAP-2 contributed to efficient recruitment of the ?TuC to the plasma membrane. Depletion of GTAP1, but not of GTAP-2, severely disrupted both the microtubule array and the honeycomb-like structure of the adult germline. We propose that GTAP-1 and GTAP-2 are unconventional components of the ?TuC that contribute to the organization of both centrosomal and non-centrosomal microtubules by targeting the ?TuC to specific subcellular sites in a tissue-specific manner.