Structural and functional analysis of a chloroplast transit peptide : interactions with the chloroplast translocation apparatus

Protein targeting into organelles is a central cellular process that occurs in allliving organisms. Proper cellular targeting is essential for the functioning of most proteins within a cell, yet the mechanism by which this process is mediated is not clearly understood. Plastids from plants provide an excellent model system for studying protein targeting as they are semi-autonomous organelles with a wide variety of structural and functional diversity. Although, plastids have their own genome they strongly rely on imported proteins that are encoded in the nuclear genome and translated in the cytoplasm. The proteins synthesized in the cytoplasm have an N-terminal extension called the transit peptide, which is considered to be necessary and sufficient for the import of proteins into plastids. Binding of precursors to the plastid surface probably involves both proteins and lipids of the envelope membrane. Several proteins in the outer and inner membrane of chloroplasts have been identified as components of the chloroplast translocation machinery. These proteins form the Toe (Iranslocon at the QUter-envelope-membrane of £.hloroplasts) components and the Tic CTranslocon at the inner-envelope-membrane of £.hloroplasts) components.