SART1 uniquely localizes to spindle poles forming a SART1 cap and promotes spindle pole assembly

The nuclear protein squamous cell carcinoma antigen recognized by T cells 1 (SART1) has been associated with pre-mRNA splicing, but SART1 RNAi knockdown results also in defects in mitotic progression, centrosome biogenesis, and chromosome cohesion. The mitotic roles of SART1 have not been characterized in detail, and it remains unclear whether SART1 functions in mitosis directly or indirectly via pre-mRNA splicing. Here, we identify SART1 as a direct, mitosis-specific microtubule-associated protein. SART1 downregulation in human cells leads to spindle assembly defects with reduced microtubule dynamics, end-on attachment defects, and checkpoint activation, while microtubule dynamics remain unaffected in interphase. SART1 uniquely localizes to the distal surface of mitotic centrosomes along the spindle axis, forming a previously not described structure we refer to as SART1 cap. Immunoprecipitation of SART1 consistently identifies centrosomal proteins as interaction partners. Immunostaining of these shows that SART1 downregulation does not affect centriole duplication and centrosome accumulation of γ-tubulin but reduces the accumulation of selective pericentriolar material (PCM) proteins such as ninein. Depletion of SART1 from frog egg extracts disrupts spindle pole assembly around sperm nuclei and DNA-coated beads. Spindles formed around DNA-coated beads do not contain centrosomes but still recruit PCM proteins for spindle pole assembly. We finally show that the N-terminus of SART1 is its microtubule-binding region and is essential for spindle assembly. Our data unravel a unique localization of SART1 and its novel function to recruit selective PCM proteins for spindle pole assembly in centrosomal and acentrosomal spindle assembly.