genetic variations and mutations) are passed only through the where they carry out their functions. Others, such as integral membrane
female line. proteins or proteins for secretion, are translocated across intracellular
Mitochondria are distributed within a cell according to regional membranes for post-translational modification and targeting to their
energy requirements, e.g. near the bases of cilia in ciliated epithelia, in destinations. This is achieved by the signal sequence, an addressing
the basal domain of the cells of proximal convoluted tubules in the system contained within the protein sequence of amino acids, which is
renal cortex (where considerable active transport occurs) and around recognized by receptors or translocators in the appropriate membrane.
the proximal segment, called middle piece, of the flagellum in sperma- Proteins are thus sorted by their signal sequence (or set of sequences
tozoa. They may be involved with tissue-specific metabolic reactions, that become spatially grouped as a signal patch when the protein folds
e.g. various urea-forming enzymes are found in liver cell mitochondria. into its tertiary configuration), so that they are recognized by and enter