Patronin/Shot Cortical Foci Assemble the Noncentrosomal Microtubule Array that Specifies the Drosophila Anterior-Posterior Axis

Abstract

SummaryNoncentrosomal microtubules play an important role in polarizing differentiated cells, but little is known about how these microtubules are organized. Here we identify the spectraplakin, Short stop (Shot), as the cortical anchor for noncentrosomal microtubule organizing centers (ncMTOCs) in the Drosophila oocyte. Shot interacts with the cortex through its actin-binding domain and recruits the microtubule minus-end-binding protein, Patronin, to form cortical ncMTOCs. Shot/Patronin foci do not co-localize with γ-tubulin, suggesting that they do not nucleate new microtubules. Instead, they capture and stabilize existing microtubule minus ends, which then template new microtubule growth. Shot/Patronin foci are excluded from the oocyte posterior by the Par-1 polarity kinase to generate the polarized microtubule network that localizes axis determinants. Both proteins also accumulate apically in epithelial cells, where they are required for the formation of apical-basal microtubule arrays. Thus, Shot/Patronin ncMTOCs may provide a general mechanism for organizing noncentrosomal microtubules in differentiated cells.

Control of apico–basal epithelial polarity by the microtubule minus-end-binding protein CAMSAP3 and spectraplakin ACF7

“…While this paper was in preparation, the Drosophila homologues of CAMSAP3 and ACF7, Patronin and Shortstop (Shot), respectively, were demonstrated to be present in the same apical complex and cooperate with each other, as well as an apical form of spectrin, in organizing apico-basal microtubule arrays in fly epithelia (Khanal et al, 2016) and in early fly embryos (Nashchekin et al, 2016). These findings are also consistent with earlier work in flies, which has demonstrated the importance of Shot for microtubule minus-end organization during tubulogenesis of the embryonic salivary glands (Booth et al, 2014).…”

Microtubule–microtubule sliding by kinesin-1 is essential for normal cytoplasmic streaming in Drosophila oocytes

“…Using the sliding-deficient Khc mutA flies, we were able to determine that KHC-driven microtubule sliding is important for proper localization of posterior determinants but is dispensable for dorsal-ventral patterning. Considering the slow, uncoordinated streaming in stage 8-9 oocytes (5,7,8), we propose that posterior determinants are initially transported along the microtubule network and are enriched and anchored at the posterior side by a local anchor (61,70,71); in stage 10B-11 oocytes, fast streaming can recycle any escaped posterior determinants by moving them along the cortex and bringing them back to the posterior anchorage sites (Fig. 6).…”

A conceptual view at microtubule plus end dynamics in neuronal axons

“…As a further mechanism, MT fragments were suggested to act as sites for re-polymerisation in axons (Baas et al, 2016). Such MT fragments could potentially be bound and directionally anchored to the axonal cortex via gigantic actin-MT linker molecules called spectraplakins (Nashchekin et al, 2016) (Section 5). …”