Question

proteins are directed to the appropriate organelles and how they are imported across the organellar membrane

Answer 1

Protein are imported across the organellar membrane , this phenomenon is known as protein targeting or protein sorting

protein targeting is the biological mechanism by which proteins are transported to their appropriate destinations in the cell or outside it. protein can be targeted to the inner space of organelle, different intra-cellular membranes, plasma membrane , or to exterior of the cell via secretion. this delivery process is carried out based on information contained in protein itself. incorrect sorting is crucial for the cell , error can lead to disease.

TARGETING SIGNALS

targeting signals are the pieces of information that enables the cellular transport machinery to correctly position a protein inside or outside the cell. this information is contained in the polypeptide chain or in the folded protein .the continuous stretch of amino acid residues in the chain that enables targeting are called signal peptides or targeting peptides.

sorting of protein in mitochondria and chloroplast both

MITOCHONDRIA

most mitochondrial protein are synthesized as cytosolic precursors containing uptake peptide signals . cytosolic chaperones deliver pre-proteins to channel linked receptor in the mitochondrial membrane . the protein with pre-sequence targeted for the mitochondria is bound by receptor and the general import pore (GIP) at the outer membrane .the protein is trans-located through TOM (trans-locase of outer membrane ) as hairpin loops. the preprotein is transported through the inter-membrane space by small TIMs (Trans-locase of inner membrane) to TIM 22 at the inner membrane

CHLOROPLAST

The preparation of chloroplast may contain stromal import sequence or a stromal and thylakoid sequence .the majority of pre-proteins are trans-located through the toc and tic complex located within the chloroplast envelope .in the stroma the stromal import sequence is cleaved off and folded as well as intra-chloroplast sorting to thylakoid continues. protein targeted to the envelope of chloroplast usually lack cleavable sorting sequence