Higher plant calreticulins have acquired specialized functions in Arabidopsis.
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.
Christensen
Anna
A
Department of Biochemistry, Center for Chemistry and Chemical Engineering, Lund University, Lund, Sweden.
Svensson
Karin
K
Thelin
Lisa
L
Zhang
Wenjing
W
Tintor
Nico
N
Prins
Daniel
D
Funke
Norma
N
Michalak
Marek
M
Schulze-Lefert
Paul
P
Saijo
Yusuke
Y
Sommarin
Marianne
M
Widell
Susanne
S
Persson
Staffan
S
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
D037282:Calreticulin; D018832:Molecular Chaperones; D011994:Recombinant Proteins; D002118:Calcium