Worm Breeder's Gazette 14(2): 32 (February 1, 1996)

These abstracts should not be cited in bibliographies. Material contained herein should be treated as personal communication and should be cited as such only with the consent of the author.

IMMUNE DEFENSE OF NEMATODES: ASABF, an antibacterial protein isolated from a nematode.

Yusuke Kato

Department of Insect Physiology and Behavior, National Institute of Sericultural and Entomological Science, Tsukuba, Ibaraki 305, Japan

        The B- and T-cell based adaptive immunity is thought to be found
only in vertebrates. In contrast, several systems categorized in innate
immunity (e.g. the defense by phagocytes, superoxides, and antibacterial
proteins etc) have been reported in many species of animals, both
vertebrates and invertebrates. It might be worthwhile if we can study the
innate immunity of nematodes, because of the utility of C. elegans.
        Previously, we reported that at least three humoral defense
activities (antibacterial, bacteriolytic, and agglutinating) were detected
in the body fluid of the parasitic nematode Ascaris suum (1). The
substantial nature of these activities was suggested to be
proteins/peptides, because all of them were lost by trypsin digestion. One
of them, ASABF (Ascaris suum antibacterial factor) is a heat-stable
antibacterial protein with molecular mass of 7 kDa. Recently, we purified
ASABF, and its amino acid sequence was determined. Eight Cys residues
which might contribute to intramolecular disulfide bonds were found in
this sequence.
        MPsrch data base searches revealed weak sequence identity between
ASABF and insect/arthropod defensins (IADs), the antibacterial proteins
which contain 6 Cys residues (2). Jumbling test supported that this
similarity was significant. All of them were positively charged proteins.
In addition, Gram-positive bacteria were much more sensitive to ASABF
(IC50=0.6 microgram/ml, against Staphylococcus aureus) than Gram-negative
bacteria (IC50=50 microgram/ml, against Esherichia coli), and this
antibacterial spectrum is similar to IADs. However, ASABF consisted of
over 60 residues, whereas IADs consisted of only 30-40 residues. The
positions of Cys residues in the sequence of IADs were highly conserved,
and they are essential to the antibacterial activity. In contrast, an
additional Cys of ASABF was found at the internal position of conserved
array of Cys residues. ASABF, therefore, is not a typical IAD, although
ASABF might be evolutionally related to IADs. 
        Many antibacterial proteins have been isolated from various insect
sources. Their gene regulation mechanisms have been explored in Drosophila
and other insects. Interestingly, similar regulatory factors which
originally found in vertebrates (e.g. NFkappaB) was also essential in
insects. Its relationship to factors involved in embryonic pattern
formation has been discussed (Dif and dorsal)(3, 4). What will be found in
the ASABF genes? The sequencing of cDNAs and genes of ASABF, and the
search for its homologues in C.elegans are currently proceeding.

(1) Kato, Y. (1995) Zool. Sci. 12, 225-230.
(2) Boman, H.G. (1995) Annu. Rev. Immunol. 13, 61-92.
(3) Ip, Y.T. et al. (1993) Cell 75, 753-763.     
(4) Hoffmann, J.A. (1995) J. Cell. Biochem. 21A, 188 (an abstract for
Keystone symposia 1995)