The presequence of Euglena LHCPII, a cytoplasmically synthesized chloroplast protein, contains a functional endoplasmic reticulum-targeting domain.
The precursor to the Euglena light-harvesting chlorophyll a/b-binding protein of photosystem II (pLHCPII) is unique; it is a polyprotein, synthesized on membrane-bound ribosomes and transported to the Golgi apparatus prior to chloroplast localization. A cDNA corresponding to the 5' end of LHCPII mRNA has been isolated and sequenced. The deduced amino acid sequence of this cDNA indicates that Euglena pLHCPII contains a 141-amino acid N-terminal extension. The N-terminal extension contains three hydrophobic domains and a potential signal peptidase cleavage site at amino acid 35. Cotranslational processing by canine microsomes removed approximately 35 amino acids from an in vitro synthesized 33-kDa pLHCPII composed of a 141-amino acid N-terminal extension and a 180-amino acid partial LHCPII unit truncated at the beginning of the third membrane-spanning hydrophobic domain. Processed pLHCPII was degraded by exogenous protease, indicating that it had not been translocated to the microsomal lumen. Extraction with 0.1 M Na2CO3, pH 11.5, did not remove the processed pLHCPII from the microsomal membrane. A stop-transfer membrane anchor sequence appears to anchor the nascent protein within the membrane, preventing translocation into the lumen. Taken together, these results provide biochemical evidence for a functional cleaved signal sequence within the N-terminal extension of a Euglena cytoplasmically synthesized chloroplast-localized protein.
Kishore
R
R
School of Biological Sciences, University of Nebraska, Lincoln 68588.
Muchhal
U S
US
Schwartzbach
S D
SD
D000595:Amino Acid Sequence; D000818:Animals; D001059:Apoproteins / Q000096:biosynthesis*; D001483:Base Sequence; D003001:Cloning, Molecular; D017124:Conserved Sequence; D004721:Endoplasmic Reticulum / Q000378:metabolism*; D005055:Euglena / Q000378:metabolism*; D007425:Intracellular Membranes / Q000378:metabolism; D045342:Light-Harvesting Protein Complexes; D008861:Microsomes / Q000378:metabolism; D008969:Molecular Sequence Data; D009838:Oligodeoxyribonucleotides; D045322:Photosynthetic Reaction Center Complex Proteins / Q000096:biosynthesis*; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D011498:Protein Precursors / Q000096:biosynthesis* / Q000378:metabolism*; D021382:Protein Sorting Signals / Q000378:metabolism; D015800:Protozoan Proteins; D012333:RNA, Messenger / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D014158:Transcription, Genetic
D001059:Apoproteins; C489396:LHCPII apoprotein, Euglena; D045342:Light-Harvesting Protein Complexes; D009838:Oligodeoxyribonucleotides; D045322:Photosynthetic Reaction Center Complex Proteins; D045332:Photosystem II Protein Complex; D010940:Plant Proteins; D011498:Protein Precursors; D021382:Protein Sorting Signals; D015800:Protozoan Proteins; D012333:RNA, Messenger