A perinuclear microtubule-organizing center controls nuclear positioning and basement membrane secretion

Abstract

Non-centrosomal microtubule-organizing centers (ncMTOCs) have a variety of roles presumed to serve the diverse functions of the range of cell types in which they are found. ncMTOCs are diverse in their composition, subcellular localization, and function. Here we report a novel perinuclear MTOC in Drosophila fat body cells that is anchored by Msp300/Nesprin at the cytoplasmic surface of the nucleus. Msp300 recruits the MT minus-end protein Patronin/CAMSAP, which functions redundantly with Ninein to further recruit the MT polymerase Msps/XMAP215 to assemble noncentrosomal MTs and does so independently of the widespread MT nucleation factor g-tubulin.Functionally, the fat body ncMTOC and the radial MT arrays it organizes is essential for nuclear positioning and for secretion of basement membrane components via retrograde dynein-dependent endosomal trafficking that restricts plasma membrane growth. Together, this study identifies a perinuclear ncMTOC with unique architecture and MT regulation properties that serves vital functions. Highlights• A novel perinuclear MTOC in differentiated fat body cells • The predominant nucleator, g-tubulin, is not required at the fat body ncMTOC • Msp300/Nesprin organizes the ncMTOC at the nuclear surface by recruiting Patronin/CAMSAP and the spectraplakin Shot • Patronin cooperates with Ninein to control MT assembly at the fat body ncMTOC by recruiting Msps • Msps, a MT polymerase, is essential for radial MT elongation from the fat body ncMTOC • Patronin and Msps associate • The ncMTOC and radial MTs, but not actin, control nuclear positioning in the fat body • The fat body MTOC controls retrograde endocytic trafficking to regulate plasma membrane growth and secretion of basement membrane proteins Here we report the discovery of an ncMTOC that is assembled on the surface of nuclei in Drosophila larval fat body cells, a differentiated cell type that has critical secretory functions and serves the metabolic needs of the organism. Assembly of this perinuclear ncMTOC requires the Msp300/Nesprin (nuclear envelope spectrin repeat protein) as a primary organizer/anchor. Msp300 recruits Patronin, which functions redundantly with Ninein for MT assembly at the ncMTOC. Patronin and Ninein cooperate to recruit Msps, which is essential for the elongation of MTs from the fat body ncMTOC. Recruitment of Msps and MT assembly at the ncMTOC are independent of g-tubulin. Functionally, this ncMTOC is necessary for: 1) the regulation of nuclear positioning, 2) controlling plasma membrane growth by facilitating dynein/Rab5-mediated retrograde endosomal trafficking of excess plasma membrane, and 3) the secretion of collagen IV and other basement membrane (BM) proteins. Together, this study identifies a unique perinuclear ncMTOC with novel MT assembly mechanisms that controls physiological roles of fat body cells by supporting nuclear positioning and vital secretory functions of fat body cells.

γ-TuRCs are required for asymmetric microtubule nucleation from the somatic Golgi of Drosophila neurons

“…These proteins bind directly to a/b-tubulin dimers and can either catalyse their assembly into microtubule polymers or stabilise nascent microtubule polymers. The extent to which γ-TuRC-independent nucleation occurs within cells remains unclear, but examples of γ-TuRC independent nucleation occurring both naturally (Coquand et al, 2021;Nashchekin et al, 2016;Tanaka et al, 2012;Wang et al, 2015;Zheng et al, 2019) and after γ-TuRC depletion (Hannak et al, 2002;Llamazares et al, 1999;Sampaio et al, 2001;Tsuchiya and Goshima, 2021) have been reported.…”