Protein targeting chaperones

A targeting chaperone involved in the transmembrane transport of proteins involves the recognition of the signal peptide. See Pool, M.R, Signal recognition particles in chloroplasts, bacteria, yeast, and mammals. Mol. Membrane Biol. 22, 3-15, 2004. 

For the most common types of biological clusters ([2Fe-2S], [3Fe-4S], and [4Fe-4S]), the assembly machinery includes iron chaperones, cysteine desulfurases, electron transfer proteins, molecular chaperones, and scaffold proteins on which the nascent cluster is assembled prior to insertion into a target protein. For the more complex and unusual clusters, such as FeMoco of nitrogenase, the [NiFe] center of hydrogenase, or the Fl-cluster of [FeFe] hydrogenase, significantly less is known about the cluster assembly process. As we shall discover below, one common theme that these systems share in the synthesis of their respective metallocofactors is the involvement of radical chemistry provided by radical SAM enzymes. 

Translocation is believed to occur posttranslation-ally, after the matrix proteins are released from the cytosolic polyribosomes. Interactions with a number of cytosolic proteins that act as chaperones (see below) and as targeting factors occur prior to translocation. 

The general types of protein-protein interactions that occur in cells include receptor-ligand, enzyme-substrate, multimeric complex formations, structural scaffolds, and chaperones. However, proteins interact with more targets than just other proteins. Protein interactions can include protein-protein or protein-peptide, protein-DNA/RNA or protein-nucleic acid, protein-glycan or protein-carbohydrate, protein-lipid or protein-membrane, and protein-small molecule or protein-ligand. It is likely that every molecule within a cell has some kind of specific interaction with a protein. 

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