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Recent evidence suggests that tyrosine kinases are highly organized in caveolae of tissue culture cells. We now report the isolation of a membrane domain from neuronal plasma membranes that has the biochemical characteristics of caveolae. A low density membrane (LDM) fraction with the same density as caveolae was highly enriched in tyrosine kinases such as insulin receptors, neurotrophin receptors, Eph family receptors, and Fyn. Grb2, Ras, heterotrimeric GTP-binding proteins, and Erk2 were also concentrated in the LDM. Incubation of the LDM fraction at 37 degrees C stimulated the phosphorylation on tyrosine of multiple, resident proteins, whereas the bulk membrane fraction was devoid of tyrosine kinase activity. The LDM, which makes up approximately 5-10% of the plasma membrane protein, appears to be organized for signal transduction.

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.

Translocation intermediates of the chimeric protein precursor Oee1-Dhfr were generated and used to identify envelope components in close proximity to the arrested precursor. The translocation of Oee1-Dhfr across the chloroplast envelope can be arrested at low ATP levels or by prebinding the fusion precursor with anti-Dhfr IgGs. The arrested Oee1-Dhfr precursor appears to span both the outer and inner envelope membranes. Translocational arrest of Oee1-Dhfr by low ATP levels was reversible, and import was restored upon resupplementation with higher ATP levels. Chemical cross-linking and co-immunoprecipitation with monospecific antibodies indicate that two outer envelope membrane proteins (Com44 and Com70) and at least one inner envelope protein (Cim44 and Cim97) were found to be in close proximity to Oee1-Dhfr during translocation. The Com70 protein was further studied and additional evidence for its role in chloroplast protein import is presented.

A cDNA clone encoding a cognate 70-kDa heat shock protein from the spinach chloroplast outer envelope (SCE70) was recently characterized (Ko, K., Bornemisza, O., Kourtz, L., Ko, Z. W., Plaxton, W. C., and Cashmore, A. R. (1992) J. Biol. Chem. 267, 2986-2993). Initial studies revealed that SCE70 is targeted to the chloroplast outer envelope membrane without further processing. To determine whether SCE70 possesses a "targeting domain," we tested the targeting ability of SCE70 proteins with various carboxyl- and amino-terminal deletions. Carboxyl-terminal deletions of up to 60% of the protein had no apparent effect on the targeting ability of SCE70. Amino-terminal deletions abolished targeting to the chloroplast except when the extreme NH2-terminal 48-amino acid sequence was retained. We further assessed the chloroplast-targeting ability of the NH2-terminal 48 amino acids by fusing to the foreign protein, mouse dihydrofolate reductase (DHFR). The resulting fusion protein, SCE70-DHFR, was localized to the outer envelope membrane of isolated chloroplasts. SCE70-DHFR exhibited targeting characteristics similar to native SCE70. The targeting of SCE70-DHFR was inhibited effectively by anti-SCE70 antibodies. Immunoprecipitation and chemical cross-linking experiments revealed that SCE70-DHFR is targeted to the same complex as SCE70 in the chloroplast envelope. These results suggest that the extreme NH2 terminus of SCE70 is required for directing SCE70 to a destination in the chloroplast outer envelope membrane, possibly through assembling the polypeptide into a protein complex.