Regulation of L-periaxin by the Ubiquitin/Proteasome pathway

Abstract

The morphological changes required for the ensheathment of peripheral nerve axons by Schwann cells are believed to be regulated by the cell cytoskeleton and its associated proteins. Control of the levels of these proteins is likely to be necessary for the assembly of compact myelin and the stability of the sheath.L-periaxin was initially identified as a putative cytoskeleton-associated protein expressed by myelin-forming Schwann cells based upon its insolubility in non-ionic detergent. The pattern of developmental expression of L-periaxin and its shift in localisation from the adaxonal to abaxonal membranes of myelinating Schwann cells following their association with axons, implied a role in the stabilisation of the myelin sheath. In this work, an F-box containing protein termed Fbxl6, was found to associate with the C-terminal acidic region of L-periaxin, in a search for binding partners of Lperiaxin using the yeast two-hybrid method. The observed interaction was verified by in vitro pull down assays using mouse sciatic nerve homogenate and L-periaxin generated by in vitro transcription/ translation.F-box proteins have been identified as components of a multi-enzyme complex termed SCF (Skp 1 / Cullin 1/F-box), which is responsible for the recruitment of substrates for ubiquitination and subsequent destruction. Fbxl6 belongs to the leucinerich repeat (LRR)-containing subfamily of F-box proteins. The C-terminal LRR region of the protein serves as the binding site for L-periaxin. whereas the F-box motif permits association with the core SCF complex. L-periaxin was detected as a ubiquitin conjugate in sciatic nerve explant cultures. Ubiquitination of the protein acts as a signal for degradation by the 26S proteasome, as revealed by stabilisation of L-periaxin upon inhibition of the proteasome by epoxomicin.The participation of L-periaxin in a recently identified dystroglycan- dystrophinrelated protein 2 (DRP2) complex, suggests an indirect role for Fbxl6 in the structural and signalling functions of the cortical cytoskeleton. Regulation of the levels of Lperiaxin by the ubiquitin/ proteasome pathway, mediated by Fbxl6, is likely to be important for the stabilisation of the Schwann cell-axon unit.