Structural Biology

Biography

Biography: Ina Buchholz

Abstract

Beta2-glycoprotein I (beta2GPI) is a soluble blood protein (326 AA, 5 domains) exhibiting two main conformational states: the circular or closed conformation, where the first domain (DI) is bound  to  the  last  domain  (DV)  of  the  protein;  and  the  linear  or  open  conformation.  In  the  open form,  beta2GPI  binds  to  phospholipid  membranes  via  DV  and  this  form  is  considered  to  play  a crucial role in the autoimmune disease antiphospholipid syndrome (APS). Therefore, investigating the  structural  dynamics   of  this  protein   is  of   high  interest.  We  investigated   different   post- translational  modifications  (PTM)  of  beta2GPI  and  studied  the  impact  on  its  conformation  with biophysical  tools  (e.g.  atomic  force  microscopy,  circular  dichroism  spectroscopy).  Additional insights into the interaction of DI and DV were gained from molecular dynamic simulation studies. PTM  1:  Lysine  residue  acetylation  reveals  a  partial  opening  of  beta2GPI  dependent  on  the acetylation ratio used (Buchholz et al., PCCP 2018). These data indicate that lysines predominantly stabilize the closed conformation and in vivo acetylation via acetyltransferases could destabilize the closed form, leading to a facilitated opening of the structure. PTM 2: Enzymatic reduction of the C- terminal  Cys288/Cys326  disulfide  bond  near  the  putative  contact  interface  of  DI  and  DV  also initiates a conformational change of beta2GPI. Furthermore, disruption of this disulfide bond leads to loosening of a 22 AA flexible loop carrying lysine residues critical for phospholipid membrane binding.  In  summary,  these  PTM  reveal  a  critical  level  of  destabilization of  the  closed  beta2GPI conformation and beta2GPI conformational change may have a large impact on APS disease.