Transposon-disruption of a maize nuclear gene, tha1, encoding a chloroplast SecA homologue: in vivo role of cp-SecA in thylakoid protein targeting.
A nuclear mutant of maize, tha1, which exhibited defects in the translocation of proteins across the thylakoid membrane, was described previously. A transposon insertion at the tha1 locus facilitated the cloning of portions of the tha1 gene. Strong sequence similarity with secA genes from bacteria, pea and spinach indicates that tha1 encodes a SecA homologue (cp-SecA). The tha1-ref allele is either null or nearly so, in that tha1 mRNA is undetectable in mutant leaves and cp-SecA accumulation is reduced > or = 40-fold. These results, in conjunction with the mutant phenotype described previously, demonstrate that cp-SecA functions in vivo to facilitate the translocation of OEC33, PSI-F and plastocyanin but does not function in the translocation of OEC23 and OEC16. Our results confirm predictions for cp-SecA function made from the results of in vitro experiments and establish several new functions for cp-SecA, including roles in the targeting of a chloroplast-encoded protein, cytochrome f, and in protein targeting in the etioplast, a nonphotosynthetic plastid type. Our finding that the accumulation of properly targeted plastocyanin and cytochrome f in tha1-ref thylakoid membranes is reduced only a few-fold despite the near or complete absence of cp-SecA suggests that cp-SecA facilitates but is not essential in vivo for their translocation across the membrane.
Voelker
R
R
Institute of Molecular Biology, University of Oregon, Eugene 97403, USA.
Mendel-Hartvig
J
J
Barkan
A
A
D000595:Amino Acid Sequence; D001483:Base Sequence; D002352:Carrier Proteins / Q000235:genetics*; D002467:Cell Nucleus; D060365:Chloroplast Proteins; D002736:Chloroplasts / Q000235:genetics*; D003001:Cloning, Molecular; D004251:DNA Transposable Elements; D018744:DNA, Plant; D017343:Genes, Plant; D008565:Membrane Proteins / Q000235:genetics*; D008969:Molecular Sequence Data; D009154:Mutation; D010940:Plant Proteins / Q000235:genetics*; D017386:Sequence Homology, Amino Acid; D003313:Zea mays / Q000235:genetics*
D002352:Carrier Proteins; D060365:Chloroplast Proteins; D004251:DNA Transposable Elements; D018744:DNA, Plant; D008565:Membrane Proteins; D010940:Plant Proteins; C021261:thylakoid polypeptides