Structure of Caldesmon in Solution

CD is an extremely long and thin molecule. A length of 75-80 nm has been estimated by direct observation in the electron microscope (Mabuchi and Wang, 

Observation by electron microscopy and nuclear magnetic resonance (nmr) spectroscopy suggests that it is made up of several rigid rod sections joined by flexible connections. One rod section, extending over 30-40 nm in the center of the molecule, is present only in CDh (Stafford et al., 1990 Mabuchi and Wang, 1991). It corresponds to the sequence coded for by exons 3b and 4 (Fig. 1). 

A four-domain model was proposed for CD from analysis of the amino acid sequence, predicted secondary structure, and proteolytic digestion patterns (Marston and Redwood, 1991). This model is compatible with the recent genomic sequence and with functional analysis. 

Domain 1 stretches from amino acids 1 to 207 (198 in chicken) and includes exons 1,2, and 3a. This domain is extended, may contain some a-helix, and contains binding sites for myosin and TM. Domain 2 stretches from 208 to 462 (431 in chicken), corresponding to exons 3b and 4, and binds to TM. This domain is absent from all CD/. 

Domain 2 is predominantly a-helical and almost entirely made up of acidic and basic amino acids in chicken CD it consists of a motif of 15 amino acids based on the sequence EEE(R/K)KAAEERERAKA, which is repeated 10 times (Bryan etal., 1989 Hayashi etal., 1989). This motif is considerably less well conserved in the human CDh sequence (Humphrey et al, 1992), how- 

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