An insight into the assembly and organization of Photosystem <span style="mso-bidi-font-weight:bold">I complex in thylakoid membranes of the thermophilic cyanobacterium, <i style="mso-bidi-font-style:normal">Mastigocladus laminosus</i> </span>

Abstract

The present study characterizes the assembly and organization of Photosystem I (PSI) complex, and its individual subuni ts into the thylakoid membranes of the thermophilic cyanobacterium, <i>Mastigocladus laminosus. </i>PSI is a multiprotein complex that contains peripheral as well as integral subunits. Hence. it serves as a suitable model system for understanding the formation and organization of membrane protein complexes. In the present study, two peripheral cytosol facing subunits of PSI. namely, PsaD and PsaE were overexpressed in <i>E. coli </i>and used for assembly studies. The gene encoding PsaK, an integral membrane spanning subunit of PSI, was cloned and the deduced amino acid sequence revealed PsaK to have two transmembrane α-helices. The characterization of the in <i>vitro </i>assembly of the peripheral subunits. PsaD and PsaE, as well as or the integral subunit, PsaK, was performed by incubating each subunit with thylakoids isolated from <i>Mostigocladus laminosus. </i>All three subunits studied were found to assemble into the thylakoids in a spontaneous mechanism, showing no requirement for eytosolic factors or NTP's (nucleotide 5'-triphosphate). Nevertheless. further characterization of the assembly of PsaK revealed its membrane integration to be most efficient at 55°C.</p>

The associations and protein-protein interactions between different subunits within the assembled PSI complex were directly quantified by measurements performed using the BIACORE technology. The preliminary results indicated the existence of specific interaction between PsaD and PsaE. and revealed a very high binding affinity between PsaD and the PSI electron acceptor ferridoxin <i>(Kd = </i>5.8 <img src='/image/spc_char/cross.gif' border=0> 10^-11<i style="mso-bidi-font-style:normal">M</i>). PsaE has exhibited a much lower binding affinity for ferridoxin <i>(Kd </i>= 3.1 <img src='/image/spc_char/cross.gif' border=0> 10^-11<i>M). </i>thereby supporting the possibility of PsaE being one of the subunits responsible for the dissociation of ferridoxin from the PSI complex.

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