A distinct isoform of Msp300 (nesprin) organizes the perinuclear microtubule-organizing center in adipocytes.

Abstract

In many cell types, disparate noncentrosomal microtubule-organizing centers (ncMTOC) replace functional centrosomes and serve the unique needs of the cell types in which they form. In Drosophila fat body cells, an ncMTOC is organized on the nuclear surface. This perinuclear ncMTOC is anchored by the nesprin Msp300. Msp300 and the spectraplakin short stop (shot) are codependent for localization to the nuclear envelope to generate the ncMTOC, where they recruit the microtubule (MT) minus-end stabilizer Patronin (CAMSAP). The Msp300 gene is complex, encoding at least 11 isoforms. Here, we show that two Msp300 isoforms, Msp300-PE and -PG, are required and one, Msp300-PE, appears sufficient to generate the ncMTOC. Loss of Msp300-PE/-PG impedes shot and Patronin localization to the nuclear surface and disrupts the MT array, endosomal trafficking, and nuclear positioning. Furthermore, upon loss of Msp300-PE and -PG, other Msp300 isoforms are retained at the nuclear surface despite the loss of nuclear positioning and MT organization, indicating that non-Msp300-PE/-PG isoforms are not sufficient to generate the ncMTOC. Msp300-PE has an unusual domain structure, including a lack of a KASH domain and very few spectrin repeats and appears, therefore, to have derived the function to generate an ncMTOC on the nuclear surface.