title: Higher plant calreticulins have acquired specialized functions in Arabidopsis.
abstract: BACKGROUND
  Calreticulin (CRT) is a ubiquitous ER protein involved in multiple cellular processes in animals, such as protein folding and calcium homeostasis. Like in animals, plants have evolved divergent CRTs, but their physiological functions are less understood. Arabidopsis contains three CRT proteins, where the two CRTs AtCRT1a and CRT1b represent one subgroup, and AtCRT3 a divergent member.
  
  
  RESULTS
  Through expression of single Arabidopsis family members in CRT-deficient mouse fibroblasts we show that both subgroups have retained basic CRT functions, including ER Ca2+-holding potential and putative chaperone capabilities. However, other more general cellular defects due to the absence of CRT in the fibroblasts, such as cell adhesion deficiencies, were not fully restored. Furthermore, in planta expression, protein localization and mutant analyses revealed that the three Arabidopsis CRTs have acquired specialized functions. The AtCRT1a and CRT1b family members appear to be components of a general ER chaperone network. In contrast, and as recently shown, AtCRT3 is associated with immune responses, and is essential for responsiveness to the bacterial Pathogen-Associated Molecular Pattern (PAMP) elf18, derived from elongation factor (EF)-Tu. Whereas constitutively expressed AtCRT1a fully complemented Atcrt1b mutants, AtCRT3 did not.
  
  
  CONCLUSIONS
  We conclude that the physiological functions of the two CRT subgroups in Arabidopsis have diverged, resulting in a role for AtCRT3 in PAMP associated responses, and possibly more general chaperone functions for AtCRT1a and CRT1b.
authors.affiliation: Department of Biochemistry, Center for Chemistry and Chemical Engineering, Lund University, Lund, Sweden.
authors.forename: Anna
authors.initials: A
authors.lastname: Christensen
authors.forename: Karin
authors.initials: K
authors.lastname: Svensson
authors.forename: Lisa
authors.initials: L
authors.lastname: Thelin
authors.forename: Wenjing
authors.initials: W
authors.lastname: Zhang
authors.forename: Nico
authors.initials: N
authors.lastname: Tintor
authors.forename: Daniel
authors.initials: D
authors.lastname: Prins
authors.forename: Norma
authors.initials: N
authors.lastname: Funke
authors.forename: Marek
authors.initials: M
authors.lastname: Michalak
authors.forename: Paul
authors.initials: P
authors.lastname: Schulze-Lefert
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Saijo
authors.forename: Marianne
authors.initials: M
authors.lastname: Sommarin
authors.forename: Susanne
authors.initials: S
authors.lastname: Widell
authors.forename: Staffan
authors.initials: S
authors.lastname: Persson
mesh_terms: D000595:Amino Acid Sequence; D000818:Animals; D017360:Arabidopsis / Q000502:physiology*; D002118:Calcium / Q000378:metabolism; D037282:Calreticulin / Q000737:chemistry / Q000235:genetics / Q000502:physiology*; D004721:Endoplasmic Reticulum / Q000378:metabolism; D005455:Fluorescent Antibody Technique; D051379:Mice; D018832:Molecular Chaperones / Q000378:metabolism; D008969:Molecular Sequence Data; D009154:Mutation; D011994:Recombinant Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013347:Subcellular Fractions / Q000378:metabolism
chemical_list: D037282:Calreticulin; D018832:Molecular Chaperones; D011994:Recombinant Proteins; D002118:Calcium