MAP6 is an intraluminal protein that induces neuronal microtubules to coil

Abstract

Neuronal activities depend heavily on microtubules, which shape neuronal processes and transport myriad molecules within them. Although constantly remodeled through growth and shrinkage events, neuronal microtubules must be sufficiently stable to maintain nervous system wiring. This stability is somehow maintained by various microtubule-associated proteins (MAPs), but little is known about how these proteins work. Here, we show that MAP6, previously known to confer cold stability to microtubules, promotes growth. More unexpectedly, MAP6 localizes in the lumen of microtubules, induces the microtubules to coil into a left-handed helix, and forms apertures in the lattice, likely to relieve mechanical stress. These features have not been seen in microtubules before and could play roles in maintaining axonal width or providing flexibility in the face of compressive forces during development.

A cryo-ET study of microtubules in axons

“…It may be that constraining the protein within the microtubule lumen induces secondary structure formation. However, when microtubules were co-polymerized with MAP6 in vitro, the globular intraluminal particles which form (Cuveillier et al, 2020) did not exactly resemble the densities we observed in DRG axons. Another possibility is that MAP6 brings together intraluminal proteins such as actin (Paul et al, 2020) or α-TAT (Coombes et al, 2016;Szyk et al, 2014) to form an ordered, ring-like structure.…”

“…Although globular MIPs were initially observed over 50 years ago (Echandia et al, 1968), their constituents remain unclear. Recent work proposed they contain microtubule associated protein 6 (MAP6, previously called N-STOP) (Cuveillier et al, 2020), which is known to promote formation of cold-stable microtubules in neurons (Guillaud et al, 1998). Analysis of mouse MAP6 showed it is highly disordered and lacks secondary structure apart from two short helices (Figure S3B).…”

Cytoskeletal organization of axons in vertebrates and invertebrates

“…There is wealth of information for many other contemporary lines of investigation, such as detailed descriptions of MT behaviors during axon branching ( Yu et al, 1994 ), of the ultrastructure of growth cones, dendrites, and synapses ( Peters et al, 1991 ), or of curious phenomena such as impressive packages of ER-derived tubular structures found in certain axons ( Andres, 1965b ; Peters et al, 1991 ). Axonal MTs of species as diverse as cockroach, lamprey, frog, toad, chick, mouse, and rats were reported to contain luminal material in form of a ∼4-nm-thick central dot or filament ( Andres, 1965a ; Burton, 1984 , 1987 ; Gonatas and Robbins, 1965 ; Lane and Treherne, 1970 ; Nixon et al, 1994 ; Rodríguez Echandía et al, 1968 ; Smith et al, 1970 , 1975 ; Wuerker and Palay, 1969 ), matching recent reports of MTs with incorporated actin filaments or MAP6/stable tubule-only peptide ( Cuveillier et al, 2020 ; Paul et al, 2019 Preprint ), of which the latter could help to explain long-term cold resistance of axonal MTs observed in vivo ( Delphin et al, 2012 ; Pannese et al, 1982 ; but see also Song et al, 2013 ). Furthermore, classical studies revealed the presence of post-translationally modified subdomains in MT lattices ( Baas and Ahmad, 1992 ; Baas and Black, 1990 ; Baas and Joshi, 1992 ), which start finding their explanations in current models of MT stability ( Baas et al, 2016 ).…”

Intrinsic and Extrinsic Factors Affecting Microtubule Dynamics in Normal and Cancer Cells

“…Additionally, structural MAPs can regulate the number of MT protofilaments [102]. Interestingly, it seems that MAP6 has unique properties and functions as it is a microtubule luminal protein and protects MTs against drug and cold-dependent destabilization [113]. With the exception of MAP4, which is a ubiquitous protein, and the expression of MAP7 in epithelial cells, the expression of structural MAPs is mainly restricted to the brain [114].…”