Molecular characterisation of 'transmembrane protein 192' (TMEM192), a novel protein of the lysosomal membrane

Transmembrane protein 192 (TMEM192) has been previously identified in proteomic analyses of lysosomal membranes. TMEM192 does not exhibit any significant homology to known protein families and possesses four potential transmembrane segments. To approach the molecular role of TMEM192, a detailed biochemical characterisation of this protein was performed. Expression constructs of fusion proteins containing TMEM192 and appended epitope tags were constructed. In HeLa cells these proteins were detected in membranes of lysosomes/late endosomes. To examine endogenous TMEM192, a TMEM192-specific antibody was generated and validated. With this antibody colocalisation of endogenous TMEM192 with lysosomal and late endosomal markers was demonstrated. Using Percoll density gradient centrifugation and immunoblotting, co-sedimentation of major portions of both TMEM192 and the lysosomal proteins LAMP-2 and cathepsin D into high-density fractions was observed. Interestingly, in contrast to many other lysosomal proteins no N-glycosylation of TMEM192 could be detected. Western blotting of reduced and non-reduced samples and co-immunoprecipitation experiments indicated TMEM192 to be a homodimer with one or more interchain disulphide bridges. TMEM192 was found to be strongly expressed in human kidney, liver, lung and pancreas tissue. The widespread tissue distribution could suggest an important role of TMEM192 for lysosomal function.