D. Jain et al., Structure of the induced antibacterial protein from tasar silkworm, Antheraea mylitta - Implications to molecular evolution, J BIOL CHEM, 276(44), 2001, pp. 41377-41382
The crystal structure of an antibacterial protein of immune origin (TSWAB),
purified from tasar silkworm (Antheraea mylitta) larvae after induction by
Escherichia cola infection, has been determined. This is the first insect
lysozyme structure and represents induced lysozymes of innate immunity. The
core structure of TSWAB is similar to c-type lysozymes and alpha -lactalbu
mins. However, TSWAB shows significant differences with respect to the othe
r two proteins in the exposed loop regions. The catalytic residues in TSWAB
are conserved with respect to the chicken lysozyme, indicating a common me
chanism of action. However, differences in the noncatalytic residues in the
substrate binding groove imply subtle differences in the specificity and t
he level of activity. Thus, conformational differences between TSWAB and ch
icken lysozyme exist, whereas functional mechanisms appear to be similar. O
n the other hand, alpha -lactalbumins and c-type lysozymes exhibit drastica
lly different functions with conserved molecular conformation. It is eviden
t that a common molecular scaffold is exploited in the three enzymes for ap
parently different physiological roles. It can be inferred on the basis of
the structure-function comparison of these three proteins having common phy
logenetic origin that the conformational changes in a protein are minimal d
uring rapid evolution as compared with those in the normal course of evolut
ion.