Structural Insights on Two Hypothetical Secretion Chaperones from Xanthomonas axonopodis pv. citri Juliana Fattori • Alessandra Prando • Leandro H. P. Assis • Ricardo Aparicio • Ljubica Tasic Published online: 28 May 2011 Ó Springer Science+Business Media, LLC 2011 Abstract Several Gram-negative bacterial pathogens have developed type III secretion systems (T3SSs) to deliver virulence proteins directly into eukaryotic cells in a process essential for many diseases. The type III secretion processes require customized chaperones with high speci- ficity for binding partners, thus providing the secretion to occur. Due to the very low sequence similarities among secretion chaperones, annotation and discrimination of a great majority of them is extremely difficult and a task with low scores even if genes are encountered that codify for small ( \ 20 kDa) proteins with low pI and a tendency to dimerise. Concerning about this, herein, we present struc- tural features on two hypothetical T3SSs chaperones belonging to plant pathogen Xanthomonas axonopodis pv. citri and suggest how low resolution models based on Small Angle X-ray Scattering patterns can provide new structural insights that could be very helpful in their anal- ysis and posterior classification. Keywords Xanthomonas axonopodis pv. citri (Xac) Á Type III secretion chaperones Á Flagellar chaperones Á Small angle X-ray scattering (SAXS) Abbreviations Xac Xanthomonas axonopodis pv. citri T3SSs Type III secretion systems T4SSs Type IV secretion systems CBD Chaperone binding domain IPTG Isopropyl b-D-1-thiogalactopyranoside Tris Tris(hydroxymethyl) aminomethane EDTA Ethylene diamine tetra acetic acid CD Circular dichroism SAXS Small Angle X-ray Scattering pI Isoelectric point 1 Introduction The Gram-negative bacterial pathogens of animals and plants, such as Xanthomonas axonopodis pv. citri, for example, have evolved a sophisticated arsenal of proteins, commonly directly injected into the host cells by one of the type III (T3SSs) and/or type IV (T4SSs) secretion systems [56]. The Xac genome [14] with 4,313 predicted proteins has around 30% of these with unknown function and classified as hypothetic. Among the hypothetical proteins, some were identified as potential secretion chaperones belonging to T3SS [1, 2] while T3SS was pointed out as crucial for bacterial pathogenicity and the initiation of disease [1, 2, 13, 24]. Besides cell-to-cell contact, molec- ular chaperones for secretion in T3SS are involved in the assembly of extracellular filaments or pili [9, 15, 44]. T3SS is comprised of more than 20 proteins [12] that form inner and outer membrane ring structures, an extracellular needle structure with pore-forming proteins at the distal tip that engage a host cell membrane, an ATPase at the base, with energetic and chaperone-effector recruitment roles, and a suite of chaperones to coordinate the assembly and J. Fattori Á A. Prando Á L. H. P. Assis Á L. Tasic (&) Chemical Biology Laboratory, Department of Organic Chemistry Institute, University of Campinas (UNICAMP), P.O. Box 6154, Campinas, SP 13083-970, Brazil e-mail: ljubica@iqm.unicamp.br R. Aparicio Structural Biology Laboratory, Department of Physical Chemistry, Chemistry Institute, University of Campinas (UNICAMP), P.O. Box 6154, Campinas, SP 13083-970, Brazil 123 Protein J (2011) 30:324–333 DOI 10.1007/s10930-011-9335-z