Putative high mobility group (HMG) non-histone chromosomal proteins from wheat germ. Isolation, characterisation and partial sequence analysis

HMG-1 enhances HMG-I/Y binding to an A/T-rich enhancer element from the pea plastocyanin gene

High-mobility-group proteins HMG-1 and HMG-I/Y bind at overlapping sites within the A/T-rich enhancer element of the pea plastocyanin gene. Competition binding experiments revealed that HMG-1 enhanced the binding of HMG-I/Y to a 31-bp region (P31) of the enhancer. Circularization assays showed that HMG-1, but not HMG-I/Y, was able to bend a linear 100-bp DNA containing P31 so that the ends could be ligated. HMG-1, but not HMG-I/Y, showed preferential binding to the circular 100-bp DNA compared with the equivalent linear DNA, indicating that alteration of the conformation of the DNA by HMG-1 was not responsible for enhanced binding of HMG-I/Y. Direct interaction of HMG-I/Y and HMG-1 in the absence of DNA was demonstrated by binding of 35S-labeled proteins to immobilized histidine-tagged proteins, and this was due to an interaction of the N-terminal HMG-box-containing region of HMG-1 and the C-terminal AT-hook region of HMG-I/Y. Kinetic analysis using the IAsys biosensor revealed that HMG-1 had an affinity for immobilized HMG-I/Y (Kd = 28 nM) similar to that for immobilized P31 DNA. HMG-1-enhanced binding of HMG-I/Y to the enhancer element appears to be mediated by the formation of an HMG-1-HMG-I/Y complex, which binds to DNA with the rapid loss of HMG-1.

HMG protein binding to an A/T-rich positive regulatory region of the pea plastocyanin gene promoter

Gel retardation assays using pea nuclear extracts have detected specific binding to regions of the promoter of the pea plastocyanin gene (petE). Several complexes which differ in sensitivity to competition with unlabelled promoter fragments and various DNA alternating copolymers, to heat treatment and to digestion with proteinase K have been detected. A protein factor, PCF1, forming one of these complexes was heat-stable and most sensitive to competition with poly(dAdT).poly(dAdT) compared to other alternating copolymers. DNase I footprinting assays showed that tracts of A/T-rich sequence within the -444 to -177 positive regulatory region of the petE promoter were protected in the presence of the pea nuclear extract. The factor PCF1 copurified with a high-mobility-group (HMG) protein preparation from pea chromatin. DNase I footprinting with the HMG protein preparation demonstrated that similar tracts of A/T-rich sequences within the promoter were protected. Southwestern-blot analysis of pea HMG proteins purified by gel filtration through Superose 12 detected a single DNA-binding species of 21 kDa. The properties of the factor PCF1 suggest that it is likely to be an HMG I protein.

Fractionation of plant and animal high mobility group chromosomal proteins by ion-exchange and reversed-phase high-performance liquid chromatography

A method for purifying wheat high mobility group (HMG) chromosomal proteins using a combination of weak cation-exchange and reversed-phase high-performance liquid chromatography (HPLC) is described. Previously reported HPLC systems devised for fractionating HMG proteins of vertebrate animals are not effective for the plant proteins. The system described here can be used for the fractionation of both plant and animal HMG proteins.

Blotting Index of Dissimilarity: use to study immunological relatedness of plant and animal High Mobility Group (HMG) chromosomal proteins

The High Mobility Group (HMG) proteins of vertebrate animals have been the subject of intensive study because of evidence that they may be structural proteins of transcriptionally active chromatin. Organisms other than vertebrate animals have chromatin proteins which meet the operational criteria of salt extractability and trichloroacetic acid solubility to be termed HMG proteins. However, because the properties of these proteins resemble those of vertebrate HMGs to varying degrees and because no definition of "HMG" based on biological function is available, a real question exists as to whether the proteins from other organisms should be considered HMGs. Because wheat HMG proteins have several biochemical properties in common with vertebrate HMGs and yet vary in other properties, we have used an immunological approach to study the relatedness of these two groups of proteins. We have raised polyclonal antibodies to the denatured wheat HMG proteins and have used an immunoblotting procedure to compare the affinities of these antibodies to the homologous wheat proteins and to the heterologous chicken HMG proteins. We have expressed the immunological relatedness of members of these two groups of proteins as the Blotting Index of Dissimilarity. This index is intended to be analogous to the Index of Dissimilarity determined by microcomplement fixation, and a direct comparison of the two procedures results in similar values. The magnitudes of the Blotting Indexes of Dissimilarity indicate that the antigenic features of the plant and animal HMG proteins have little in common.