EZH2 promotes a bi-lineage identity in basal-like breast cancer cells.

The mechanisms regulating breast cancer differentiation state are poorly understood. Of particular interest are molecular regulators controlling the highly aggressive and poorly differentiated traits of basal-like breast carcinomas. Here we show that the Polycomb factor EZH2 maintains the differentiation state of basal-like breast cancer cells, and promotes the expression of progenitor associated and basal-lineage genes. Specifically, EZH2 regulates the composition of basal-like breast cancer cell populations by promoting a 'bi-lineage' differentiation state, in which cells co-express basal- and luminal-lineage markers. We show that human basal-like breast cancers contain a subpopulation of bi-lineage cells, and that EZH2-deficient cells give rise to tumors with a decreased proportion of such cells. Bi-lineage cells express genes that are active in normal luminal progenitors, and possess increased colony-formation capacity, consistent with a primitive differentiation state. We found that GATA3, a driver of luminal differentiation, performs a function opposite to EZH2, acting to suppress bi-lineage identity and luminal-progenitor gene expression. GATA3 levels increase upon EZH2 silencing, mediating a decrease in bi-lineage cell numbers. Our findings reveal a novel role for EZH2 in controlling basal-like breast cancer differentiation state and intra-tumoral cell composition.

The assembly of the PsaD subunit into the membranal photosystem I complex occurs via an exchange mechanism.

PsaD is a peripheral stromal-facing subunit of photosystem I (PSI), a multisubunit complex of the thylakoid membranes. PsaD plays a major role in both the function and assembly of PSI. Past studies with radiolabeled PsaD indicated that PsaD is able to assemble in vitro specifically into the PSI complex. To unravel the mechanism by which this assembly takes place, the following steps were taken. (i) Mature PsaD of spinach and PsaD of the prokaryotic caynobacterium Mastigocladus laminosus, both bearing a six-histidine tag at their C-termini, were overexpressed in Escherichia coli and purified to homogeneity. (ii) The purified recombinant protein was introduced into the isolated PSI complex. (iii) Following incubation, the PsaD that assembled into PSI was separated from the nonassembled PsaD by a sucrose gradient. Differential Western blot analysis was used to determine whether the native and the recombinant PsaD were present as free or assembled proteins of the PSI complex. Antibodies that can recognize only the recombinant PsaD (anti-his) or both the native and recombinant PsaD (anti-PsaD) were used. The findings indicated that an exchange mechanism enables the assembly of a newly introduced PsaD into PSI. The latter replaces the PsaD subunit that is present in situ within the complex. In vivo studies that followed the assembly of PsaD in Chlamydomonas reinhardtii cells supported this in vitro-characterized exchange mechanism. In C. reinhardtii, in the absence of synthesis and assembly of new PSI complexes, newly synthesized PsaD assembled into pre-existing PSI complexes.

An insight into the assembly and organization of photosystem I complex in the thylakoid membranes of the thermophilic cyanobacterium, Mastigocladus laminosus.

The present study characterizes the assembly and organization of Photosystem I (PSI) complex, and its individual subunits into the thylakoid membranes of the thermophilic cyanobacterium, Mastigocladus laminosus. 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 E. coli 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 alpha-helices. The characterization of the in vitro assembly of the peripheral subunits, PsaD and PsaE, as well as of the integral subunit, PsaK, was performed by incubating each subunit with thylakoids isolated from Mastigocladus laminosus. All three subunits studied were found to assemble into the thylakoids in a spontaneous mechanism, showing no requirement for cytosolic 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 degrees C. 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 (Kd = 5.8 x 10(-11) M). PsaE has exhibited a much lower binding affinity for ferridoxin (Kd = 3.1 x 10(-5) M), thereby supporting the possibility of PsaE being one of the subunits responsible for the dissociation of ferridoxin from the PSI complex.