Mechanisms Of Protein Import Research Articles

Mitochondrial protein import: Bypass of proteinaceous surface receptors can occur with low specificity and efficiency

Proteolytic degradation of receptor sites on the mitochondrial surface strongly reduces the efficiency of mitochondrial protein import. The remaining residual import still involves basic mechanisms of protein import, including: insertion of precursors into the outer membrane, requirement for ATP and a membrane potential, and translocation through contact sites between both membranes. The import of a chloroplast protein into isolated mitochondria which occurs with a low rate is not inhibited by a protease-pretreatment of mitochondria, indicating that this precursor only follows the bypass pathway. The low efficiency of bypass import suggests that this unspecific import does not disturb the uniqueness of mitochondrial protein composition. We conclude that mitochondrial protein import involves a series of steps in which receptor sites appear to be responsible for the specificity of protein uptake.

Import of proteins into the chloroplast lumen.

Plastocyanin is a nuclear-encoded protein that is functional in the thylakoid lumen of the chloroplast. It is synthesized in the cytoplasm as a precursor with an N-terminal transit peptide of 66 amino acids. Its transport route involves two steps, import into the chloroplasts and subsequent routing over the thylakoid membrane into the lumen. Concomitant with the transport, the transit peptide is removed in two successive steps. The transit peptide consists of two functionally different domains. In this study we examine to what extent each domain is involved in import and routing and how far these two processes are linked. For this purpose we made deletions in the N-terminal and C-terminal part of the transit peptide and fusion proteins which only contain one of these parts. The results show that the N-terminal part of the transit peptide is responsible for import into the chloroplast. The N-terminal 43 amino acids are sufficient to direct other proteins into the stroma. The C-terminal part of the transit peptide is a prerequisite for routing inside the chloroplast but not for import. When deletions are made in this part, the transport of plastocyanin stops after import and the intermediate accumulates in the stroma or on the outside of the thylakoids. Transgenic tomato plants that constitutively express a foreign plastocyanin gene were used to study protein transport in different tissues. Normally, expression of endogenous plastocyanin genes in plants is restricted to photosynthetic tissues only. However, in the transgenic plants this foreign plastocyanin protein is found in all tissues examined. The protein is transported into the local plastids of these tissues and it is processed to the mature size. We conclude that plastids of developmentally different tissues are capable of importing precursor proteins that are normally not found in these tissues. Most likely such plastids, though functionally and morphologically differentiated, have similar or identical protein import mechanisms when compared to the chloroplasts in green tissue. The precursor of ferredoxin was expressed in Escherichia coli. Surprisingly the precursor interacts with the cytoplasmic membrane and is translocated across this membrane. The unprocessed precursor accumulates in the periplasm.

In vivo import of plastocyanin and a fusion protein into developmentally different plastids of transgenic plants

Transgenic tomato plants that constitutively express a foreign plastocyanin gene were used to study protein transport in different tissues. Normally expression of endogenous plastocyanin genes in plants is restricted to photosynthetic tissues only, whereas this foreign plastocyanin protein is found to be present in all tissues examined. The protein is transported into the local plastids in these tissues and it is processed to the mature size. We conclude that plastids of developmentally different tissues are capable of importing precursor proteins that are normally not found in these tissues. Most likely such plastids, though functionally and morphologically differentiated, have similar or identical protein import mechanisms when compared to the chloroplasts in green tissue.