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In a compartmentalized cell, correct protein localization is crucial for function of virtually all cellular processes. From the cytoplasm as a starting point, proteins are imported into organelles by specific targeting signals. Many proteins, however, act in more than one cellular compartment. In this chapter, we discuss mechanisms by which proteins can be targeted to multiple organelles with a focus on a novel gene regulatory mechanism, functional translational readthrough, that permits multiple targeting of proteins to the peroxisome and other organelles. In mammals, lactate and malate dehydrogenase are the best-characterized enzymes whose targeting is controlled by functional translational readthrough.

The multi-C2 domain protein dysferlin localizes to the plasma membrane and the T-tubule system in skeletal muscle, however, its physiological mode of action is unknown. Mutations in the DYSF gene lead to autosomal recessive limb-girdle muscular dystrophy type 2B and Miyoshi myopathy. Here we show that dysferlin has membrane tubulating capacity and that it shapes the T-tubule system. Dysferlin tubulates liposomes, generates a T-tubule-like membrane system in non-muscle cells, and links the recruitment of phosphatidylinositol 4,5-bisphosphate to the biogenesis of the T-tubule system. Pathogenic mutant forms interfere with all of these functions indicating that muscular wasting and dystrophy are caused by the dysferlin mutants' inability to form a functional T-tubule membrane system.