User:Eran Hodis/Sandbox3

General Structure

There are two distinct classes of HMGRs, class I, which is only found in eukaryotes and are membrane bound and class II, which is found in prokaryotes and are soluble. ^[1] HMGR contains 8 transmembrane domains that have yet to be successfully crystallized, which anchor the protein to the membrane of the endoplasmic reticulum. ^[2] The catalytic portion of human HMGR forms a tetramer, with the individual monomers winding around each other. ^[2] Within the tetramer, the monomers are arranged into , each of which contains which are formed by residues form both monomers. Each monomer contains , the , the , and the . The L-domain is unique to HMGRs while the S-domain, which forms the binding site for NADP, resembles that of ferredoxin. The S and L domains are connected by a which is essential for the HMG-binding site. ^[2] Salt bridges between residues R641 and E782 as well as between E700 and E700 on neighboring monomers compliment the largely hydrophobic dimer-dimer interface. ^[2]

Introduction

The nucleosome core particle contains two copies of each histone protein (H2A, H2B, H3 and H4) and 146 basepairs (bp) of superhelical DNA wrapped around this histone octamer. It represents the first order of DNA packaging in the nucleus and as such is the principal structure that determines DNA accessibility.

The Histone Octamer

Two copies of each histone protein, H2A, H2B, H3, and H4, are assembled into an octameric disc. Although each monomer has an N-terminal tail that projects from the histone core, the structure at left shows only a single tail from one H3 monomer.

All four histone proteins share a highly similar structural motif, the histone fold, comprising three alpha helices connected by two loops (shown here for H2A):

alpha1-loop1-alpha2-loop2-alpha3