Identification of heat shock protein hsp70 homologues in chloroplasts

Cytoplasmic members of the heat shock protein hsp70 family have recently been implicated in the transport of proteins to the endoplasmic reticulum and mitochondria. In addition, other hsp70 homologues have been found in the endoplasmic reticulum and mitochondria and, at least for the endoplasmic reticulum hsp70 homologue, may be involved in the proper folding and assembly of newly transported proteins. Since chloroplasts are an important site of protein transport in plant cells, we were interested in determining whether hsp70 proteins might be located in this organelle. By using immunoblotting techniques and two antibody preparations against hsp70 proteins, we have identified three chloroplastic proteins of approximately 70 kDa that are related to hsp70 proteins. One of these proteins was tightly associated with the outer envelope membrane and was not exposed at the outer surface of the chloroplasts. The other two were soluble proteins located in the stroma. Steady-state levels of the chloroplastic hsp70 homologues did not change after heat stress nor were any additional hsp70 homologues detected in chloroplasts isolated from heat-stressed plants. We discuss the possible functions of these hsp70 homologues in the transport of proteins into and within chloroplasts.