Calerythrin is a 20 kDa calcium-binding protein
isolated from gram-positive bacterium Saccharopolyspora
erythraea. Based on amino acid sequence homology,
it has been suggested that calerythrin belongs to the family
of invertebrate sarcoplasmic EF-hand calcium-binding proteins
(SCPs), and therefore it is expected to function as a calcium
buffer. NMR spectroscopy was used to obtain structural
information on the protein in solution. Backbone and side
chain 1H, 13C, and 15N
assignments were obtained from triple resonance experiments
HNCACB, HN(CO)CACB, HNCO, CC(CO)NH, and [15N]-edited
TOCSY, and HCCH-TOCSY. Secondary structure was determined
by using secondary chemical shifts and characteristic NOEs.
In addition, backbone N-H residual dipolar couplings were
measured from a spin-state selective [1H,
15N] correlation spectrum acquired from
a sample dissolved in a dilute liquid crystal. Four EF-hand
motifs with characteristic helix-loop-helix patterns were
observed. Three of these are typical calcium-binding EF-hands,
whereas site 2 is an atypical nonbinding site. The global
fold of calerythrin was assessed by dipolar couplings.
Measured dipolar couplings were compared with values calculated
from four crystal structures of proteins with sequence
homology to calerythrin. These data allowed us to recognize
an overall similarity between the folds of calerythrin
and sarcoplasmic calcium-binding proteins from the sandworm
Nereis diversicolor and the amphioxus Branchiostoma
lanceolatum.