The structure of PilA from Acinetobacter baumannii AB5075 suggests a mechanism for functional specialization in Acinetobacter type IV pili

Authors

Leslie A. Ronish, University of Nebraska-LincolnFollow
Erik Lillehoj, University of Maryland School of Medicine
James K. Fields, University of Maryland School of Medicine
Eric J. Sundberg, University of Maryland School of Medicine
Kurt Piepenbrink, University of Nebraska - LincolnFollow

ORCID IDs

https://orcid.org/0000-0002-2527-0301 Kurt Piepenbrink

Date of this Version

2019

Citation

J. Biol. Chem. (2019) 294(1) 218–230

Comments

© 2019 Ronish et al. Published under exclusive license by The American Society for Biochemistry and Molecular Biology, Inc. Used by permission.

Abstract

Type IV pili (T4P) are bacterial appendages composed of protein subunits, called pilins, noncovalently assembled into helical fibers. T4P are essential, in many bacterial species, for processes as diverse as twitching motility, natural competence, biofilm or microcolony formation, and host cell adhesion. The genes encoding type IV pili are found universally in the Gram-negative, aerobic, nonflagellated, and pathogenic coccobacillus Acinetobacter baumannii, but there is considerable variation in PilA, the major protein subunit, both in amino acid sequence and in glycosylation patterns. Here we report the X-ray crystal structure of PilA from AB5075, a recently characterized, highly virulent isolate, at 1.9 Å resolution and compare it to homologues from A. baumannii strains ACICU and BIDMC57, which are C-terminally glycosylated. These structural comparisons revealed that PilA_AB5075 exhibits a distinctly electronegative surface chemistry. To understand the functional consequences of this change in surface electrostatics, we complemented a ΔpilA knockout strain with divergent pilA genes from ACICU, BIDMC57, and AB5075. The resulting transgenic strains showed differential twitching motility and biofilm formation while maintaining the ability to adhere to epithelial cells. PilA_AB5075 and PilA_ACICU, although structurally similar, promote different characteristics, favoring twitching motility and biofilm formation, respectively. These results support a model in which differences in pilus electrostatics affect the equilibrium of microcolony formation, which in turn alters the balance between motility and biofilm formation in Acinetobacter.