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Journal of Bacteriology, November 1999, p. 6720-6729, Vol. 181, No. 21
0021-9193/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Department of Biological Chemistry,
Received 10 June 1999/Accepted 24 August 1999
A novel cellulosomal scaffoldin gene, termed cipV, was
identified and sequenced from the mesophilic cellulolytic anaerobe Acetivibrio cellulolyticus. Initial identification of the
protein was based on a combination of properties, including its high
molecular weight, cellulose-binding activity, glycoprotein nature, and
immuno-cross-reactivity with the cellulosomal scaffoldin of
Clostridium thermocellum. The cipV gene is
5,748 bp in length and encodes a 1,915-residue polypeptide with a
calculated molecular weight of 199,496. CipV contains an N-terminal
signal peptide, seven type I cohesin domains, an internal family III
cellulose-binding domain (CBD), and an X2 module of unknown function in
tandem with a type II dockerin domain at the C terminus. Surprisingly,
CipV also possesses at its N terminus a catalytic module that belongs
to the family 9 glycosyl hydrolases. Sequence analysis indicated the
following. (i) The repeating cohesin domains are very similar to each
other, ranging between 70 and 90% identity, and they also have about 30 to 40% homology with each of the other known type I scaffoldin cohesins. (ii) The internal CBD belongs to family III but differs from
other known scaffoldin CBDs by the omission of a 9-residue stretch that
constitutes a characteristic loop previously associated with the
scaffoldins. (iii) The C-terminal type II dockerin domain is only the
second such domain to have been discovered; its predicted "recognition codes" differ from those proposed for the other known dockerins. The putative calcium-binding loop includes an unusual insert, lacking in all the known type I and type II dockerins. (iv) The
X2 module has about 60% sequence homology with that of C. thermocellum and appears at the same position in
the scaffoldin. (v) Unlike the other known family 9 catalytic modules
of bacterial origin, the CipV catalytic module is not accompanied by a
flanking helper module, e.g., an adjacent family IIIc CBD or an
immunoglobulin-like domain. Comparative sequence analysis of the CipV
functional modules with those of the previously sequenced scaffoldins
provides new insight into the structural arrangement and phylogeny of
this intriguing family of microbial proteins. The modular organization of CipV is reminiscent of that of the CipA scaffoldin from C. thermocellum as opposed to the known scaffoldins from the
mesophilic clostridia. The phylogenetic relationship of the different
functional modules appears to indicate that the evolution of the
scaffoldins reflects a collection of independent events and mechanisms
whereby individual modules and other constituents are incorporated into the scaffoldin gene from different microbial sources.
Israel Institute of Technology,
*
Corresponding author. Mailing address: Department of
Biological Chemistry, The Weizmann Institute of Science, Rehovot,
Israel. Phone: (972)-8-934-2373. Fax: (972)-8-946-8256. E-mail:
bfbayer{at}weizmann.weizmann.ac.il.
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