A photoactivatable synthetic transit peptide labels 30 kDa and 52 kDa polypeptides of the chloroplast inner envelope membrane

Isolation and characterization of a cDNA clone encoding a member of the Com44/Cim44 envelope components of the chloroplast protein import apparatus

Many of the proteins in the chloroplast envelope play an important role in facilitating the biochemical and transport processes of the compartment. For the transport of proteins into the chloroplast, we have recently identified at least three different envelope proteins (Com44/Cim44, Com70, and Cim97) in close physical proximity to a partially translocated chimeric precursor protein (Wu, C., Seibert, F. S., and Ko, K. (1994) J. Biol. Chem. 269, 32264-32271). In this study we report the characterization of a cDNA clone encoding a member of the Com44/Cim44 envelope proteins. The combined data from nucleotide sequencing, and RNA and protein blot analyses indicate the existence of multiple forms of the 44-kDa envelope protein. Depending on the plant species examined, immunologically-related protein bands with molecular masses of 42 to 46 kDa were observed. Organelle subfractionation, protease treatment, and immunomicroscopic studies together provide an indication that the immunologically-related proteins may be present in both the outer and inner envelope membranes. Co-migration of the product synthesized from the cDNA insert with a 44-kDa immunoreactive band of the chloroplast envelope, and the in vitro import results, together suggest that the in vitro synthesized 44-kDa protein is targeted to the envelope membrane without any further processing.

Protein translocation across chloroplast envelope membranes

Nuclear-encoded chloroplast proteins are imported from the cytosol into the chloroplast stroma by a common translocation machinery. Several components of the import apparatus, including GTP-binding proteins and Hsp70 proteins, have recently been identified and characterized. This review discusses the role of these proteins in chloroplast protein import.

Isolation of components of the chloroplast protein import machinery

Components of the protein import machinery of the chloroplast were isolated by a procedure in which the import machinery was engaged in vitro with a tagged import substrate under conditions that yielded largely chloroplast envelope-bound import intermediates. Subsequent detergent solubilization of envelope membranes showed that six envelope polypeptides copurified specifically and, apparently, stoichiometrically with the import intermediates. Four of these polypeptides are components of the outer membrane import machinery and are associated with early import intermediates. Two of these polypeptides have been characterized. One is a homolog of the heat shock protein hsp70; the other one is a channel-protein candidate.

Kinetic analysis of translocation into isolated chloroplasts of the purified ferredoxin precursor

Time courses of the import into isolated chloroplasts of the purified ferredoxin precursor were measured at different protein concentrations. Analysis of the initial import rates indicates the presence of one saturable import system with an apparent Km value of approximately 100 nM and a Vmax corresponding to the uptake of approximately 2.5 x 10(4) precursor proteins per minute per chloroplast. We conclude that the in vitro observed activity of the chloroplast protein import machinery, functioning independently from cytosolic factors, would be enough to allow chloroplast development at physiological rate.

Lipid-peptide interactions between fragments of the transit peptide of ribulose-1,5-bisphosphate carboxylase/oxygenase and chloroplast membrane lipids

The interactions of fragments of the transit peptide of ribulose-1,5-bisphosphate carboxylase/oxygenase with lipid monolayers was studied in order to investigate the possible involvement of the membrane lipids in the protein import process. The fragments are surface active and have a differential ability to insert in lipid monolayers. The fragments have a preference for the chloroplast galacto- and sulpholipids and phosphatidylglycerol and interact with envelope membrane lipid extracts. These results suggest that probably transit peptide-lipid interactions are involved in the chloroplast protein import process.

Identification and characterization of nuclear location signal-binding proteins in nuclear envelopes

A radioiodinated, photoactivable synthetic nonapeptide corresponding to the nuclear location signal (NLS) of SV40 large T antigen has been used in photolabelling reactions with purified mouse liver nuclei, nuclear envelopes and other cellular fractions, to identify specific NLS-binding proteins which may be involved in selective transport of karyophilic proteins. SDS-polyacrylamide gel analysis of photolabelled products demonstrates that a 60 kDa nuclear protein and four nuclear envelope proteins (67, 60, 53 and 47 kDa) bind specifically to the native NLS and not to a mutant NLS or unrelated sequences. This binding shows saturation kinetics, with highest affinity of the NLS for the 60 and 67 kDa proteins. The nuclear 60 kDa NLS-binding protein is identical to the nuclear envelope 60 kDa NLS-binding protein by two-dimensional gel analysis of labelled proteins. Biochemical fractionation of labelled nuclear envelopes suggests that the 53 and 47 kDa proteins are peripheral membrane proteins whereas the 67 and 60 kDa proteins can be localized to the pore complex. The NLS also binds to solubilized 67, 60, 53 and 47 kDa proteins but with decreased affinity. Our results suggest that one of the early steps in selective nuclear transport of proteins may be the recognition of the NLS by the 60 kDa and/or 67 kDa binding proteins present in the nuclear pore complex.

Co-expression of a precursor and the mature protein of wheat ribulose-1,5-bisphosphate carboxylase small subunit from a single gene in Escherichia coli

The cDNA encoding a precursor of wheat ribulose-1,5-bisphosphate carboxylase/oxygenase was inserted in-phase with prokaryotic expression elements in four different vectors. Five expression vectors encoding the small subunit precursors were cloned in Escherichia coli. None of these constructs expressed detectable amounts of the precursor protein, but all directed synthesis of the mature small subunit. The expression of the small subunit was a consequence of an independent, intragenic Shine-Dalgarno sequence optimally located upstream from an ATG specifying the first codon of the mature small subunit portion in the precursor transcript. Similar internal translation signals have been identified in the nuclear-encoded cDNAs of the small-subunit precursors of numerous higher plant genes. The 5' end of the wheat small-subunit precursor was linked with a consensus E. coli DNA sequence such that the modified gene encoded a partial hybrid precursor carrying four additional residues at its amino terminus. The resultant construct, pEI-W3, directed abundant synthesis of both the partially hybrid small-subunit precursor and the mature small subunit, constituting as much as 10% of the total bacterial protein. The bacterially synthesized small subunit precursor was purified to homogeneity. The authenticity of the recombinant protein was verified by its size, immunological properties, amino-terminal sequence, and amino acid composition.

Phosphorylation of a 51-kDa envelope membrane polypeptide involved in protein translocation into chloroplasts

In this report we demonstrate that a 51-kDa outer-envelope membrane protein (P51) is involved in protein translocation into chloroplasts. Furthermore it is shown that phosphorylation of P51 is functionally related to the process of binding and/or importing precursor proteins into chloroplasts. Several lines of evidence have been obtained supporting this suggestion. First, protein import into chloroplasts was inhibited by the membrane-impermeable agent pyridoxal 5'-phosphate, which has been shown to react with a component of the protein-import apparatus. Phosphorylation of envelope membrane polypeptides using [gamma-32P]ATP in the presence of pyridoxal 5'-phosphate resulted in an increased incorporation of 32P radiolabel into a 51-kDa membrane polypeptide (P51). A close correlation between the inhibition of protein import and the increase in the phosphorylation state of P51, both as a function of PLP concentration, was observed. Second, binding of purified precursor proteins to chloroplasts resulted in a specific increase in the phosphorylation state of P51. This effect was not exerted by the mature form of the precursor protein lacking the presequence. Third, internally generated ATP was able to compete specifically with externally added [gamma-32P]ATP for the phosphorylation of P51. Fourth, digestion of the outer-envelope membrane with low amounts of thermolysin resulted in a loss of protein import activity, which was associated with the removal of the phosphorylation site of P51. Phosphorylation of P51 proceeds with an apparent Km (ATP) of about 5 microM, which is much lower than the ATP concentration required for the protein translocation itself. We suggest that two different ATP-dependent processes are involved in protein translocation into chloroplasts. P51 represent presumably a regulatory component of the protein-import apparatus or the protein receptor itself.