Bundling of microtubules by synapsin 1. Characterization of bundling and interaction of distinct sites in synapsin 1 head and tail domains with different sites in tubulin.

Abstract

Synapsin 1 is a nerve terminal phosphoprotein whose role seems to encompass the linking of small synaptic vesicles to the cytoskeleton. Synapsin 1 can join small synaptic vesicles to neuronal spectrin, microfilaments and microtubules; it can also bundle microtubules and microfilaments. In this paper, the mode of interaction between synapsin 1 and microtubules has been investigated. Bundling is shown to be highly cooperative: the apparent Hill coefficient is 3.06 +/- 0.3, and bundling is half-maximal at 0.63 +/- 0.02 microM. Bundling occurs either when whole synapsin 1 preparations (containing monomers and oligomers) or when monomeric synapsin 1 is added to microtubules. However, it is not clear that synapsin 1 remains monomeric in the presence of microtubules. Synapsin 1-microtubule mixtures contain two types of filament. One type is characterised by microtubules often with synapsin 1 bound to their surface. The other type is composed of filaments of diameter 15 +/- 5 nm. This filament type is granular and made up in part of 14-nm-diameter particles. These dimensions are consistent with their being made up of polymerised synapsin 1. It is possible that microtubules induce the polymerisation of synapsin 1. Synapsin 1 had independent tubulin binding sites in the N-terminal head domain and in the C-terminal tail domain. Whole synapsin 1 can interact with tubulin after it has been digested to remove the tubulin C terminus (des-C-terminal tubulin). The interaction of des-C-terminal tubulin with synapsin 1 appears to be via the head domain, since 125I-des-C-terminal tubulin only shows specific binding to the head domain on gel blots. By contrast intact tubulin binds to both head and tail domains. Binding to the tail domain can be inhibited by a synthetic peptide representing the microtubule-associated protein 2 (MAP2) binding site of class II beta tubulin. These results suggest a model for microtubule bundling by synapsin 1 in which independent sites in the head and tail domains of synapsin 1 cross-link microtubules by interactions with two distinct sites in tubulin.