The question whether insect cells do sialylate and if so, when and
what, is indeed a rather hot topic. The situation is not quite simple.
There are a few reports claiming sialylation of O- or N- or something-
glycans.
Roth et al. (Science 256, p. 673 found a polysialylated protein
in Drosophila larvae.
Vandenbroeck et al (Lymphokine Cytokine Res. 13, p. 253..) found
complex N-glycans. Sridhar et al. (FEBS Lett. 315, p. 282.) and Hafer
and Ferenz (Comp. Biochem. Physiol. 100B, p. 579..) detected sialic
acid on their recombinant glycoproteins. These three studies were done
with lectins and/or neuraminidase and/or enzymatic determination of
sialic acid. Such methods have their merits, however, they must be
rated as unreliable to some extent.
And there are the studies of Dr. Castellino and coworkers mentioned by
R. Schnaar. They have in fact received a lot of attention and might
well be citation classics in the field. There are however a few
problems with his results.
1.) For at least four years now, nobody else could confirm these
results - although a lot of people tried.
2.) To my knowledge, Castellino has not undertaken to directly
communicate with other researchers in the field on this topic. Does he
know why all the others did not ´succeed´ ? Does he use another
baculovirus or a different cell line than all the others ?
I would very much appreciate to hear or read his current opinion.
3.) My limited knowledge of cell biology does not provide me with an
idea how the expression of recombinant plasminogen might influence the
expression of non-transcribed or non-translated proteins. Anyway, all
other baculovirus constructs failed to effect such changes of genetic
regulation.
Therefore, the nasty suspicion might arise that there´s something
wrong with Castellino´s studies. He is cordially invited to dissipate
such doubts.
Apart from that I want to suggest that whenever a sialylated insect
glycoprotein seems to appear, check it´s identy, don´t rely on
lectin blots or enzymatic methods alone. Cooperate with groups
experienced in glycan analysis (to make it clear: we are no sialylation
experts) and do not rely on commercial kits (which are like tarts:
expensive but you often won´t get what you want).
_____________________________________________________________________
Altmann Friedrich
Universtitaet fuer Bodenkultur Vienna
A-1180 Austria
email: altmann at edv1.boku.ac.at
