Review
Evolutionary perspective on annexin calcium-binding domains
R.O. Morgan
*
, S. Martin-Almedina, J.M. Iglesias, M.I. Gonzalez-Florez, M.P. Fernandez
Department of Biochemistry and Molecular Biology, Edificio Santiago Gascon, Faculty of Medicine, University of Oviedo, 33006 Oviedo, Spain
Received 7 September 2004; accepted 13 September 2004
Available online 22 September 2004
Abstract
Molecular systematic analysis of the annexin gene superfamily characterized the evolutionary origin, frequency and range of structural
variation in calcium interaction domains that are considered intrinsic for membrane targeting and ion channel function. Approximately 36%
of annexin repeat domains in an estimated 100 distinct subfamilies contained amino acid changes consistent with the functional loss of type
two calcium-binding sites. At least 11% of annexin domains contained a novel K/H/RGD motif conserved in particular subfamilies and
manifest in all phyla, apparently via convergent evolution. The first yeast annexin from Yarrowia lipolytica was classified in the ANXC1
subfamily with fungal and mycetozoan representatives. This clade had intact calcium-binding sites but disruption of the normally well-
conserved, mid-repeat 4 region implicated in calcium channel regulation. Conversely, a tandem pair of novel annexins from the amphioxus
Branchiostoma floridae resembled annexin A13 in gene structure and conserved the charged amino acids associated with the internal
hydrophilic pore, but were devoid of external type 2 calcium-binding sites and incorporated K/RGD motifs instead, like annexin A9. The
selective erosion of calcium-binding sites in annexin domains and the occurrence of alternate ligands in the same exposed, interhelical loops
are pervasive features of the superfamily. This suggests greater complexity than previously appreciated in the mechanisms controlling
annexin membrane interaction and calcium channel operation.
D 2004 Elsevier B.V. All rights reserved.
Keywords: Annexin; Calcium-binding domain; Hidden Markov model; Molecular evolution; Phylogenetic analysis; Protein modeling
1. Introduction
Protein domains are conserved structural elements,
generally associated with a perceived function or mecha-
nism of interaction. Even where such a reductionist concept
may be coherent with proteomic analysis, it should be
tempered by the realization that individual domains and
integral proteins may exhibit a broad range of diversity and
specificity in both structure and function. This is achieved
through sometimes subtle structural variations, context-
sensitive differences in domain architecture, target-condi-
tioned conformational changes and host status affecting
gene expression, subcellular localization, posttranslational
modification, and pathophysiology. Molecular systematic
examination of homologous proteins can greatly aid in the
characterization of protein domains and facilitate inferences
or predictions about their mechanism and function, even
when empirical data are lacking. The basis of this approach
is that functional constraints ordain the conservation of
residues that are vital for structure and function, be they
universal or class-specific. The methodology involves
determining the evolutionary relationships among homolo-
gous members, defining molecular profiles for individual
clades or subfamilies, and mapping site-specific changes
into three-dimensional structures to infer which sites are
responsible for the preservation or divergence of function.
The diversity of calcium-binding proteins is further
complicated by the independent evolution of some protein
domains (e.g., C2 and EF-hand) within multiple, distinct
gene families [1,2]. The identification of calcium-inde-
pendent members in virtually all families of calcium-
binding proteins argues for roles that extend beyond mere
targeting domains, to proffer alternative mechanisms that
accomplish the same function, or to introduce accessory
features that modify function. The annexin superfamily
0167-4889/$ - see front matter D 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.bbamcr.2004.09.010
* Corresponding author. Tel.: +34 985 104214; fax: +34 985 103157.
E-mail address: romorgan@bioquimica.uniovi.es (R.O. Morgan).
Biochimica et Biophysica Acta 1742 (2004) 133 – 140
http://www.elsevier.com/locate/bba