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Solution structure of human sorting nexin 22

Center for Eukaryotic Structural Genomics, Department of Biochemistry, University of Wisconsin-Madison, Madison, Wisconsin 53706-1544, USA

(RECEIVED January 1, 2007; FINAL REVISION January 31, 2007; ACCEPTED February 7, 2007)

The sorting nexins (SNXs) constitute a large group of PX domain-containing proteins that play critical roles in protein trafficking. We report here the solution structure of human sorting nexin 22 (SNX22). Although SNX22 has <30% sequence identity with any PX domain protein of known structure, it was found to contain the / fold and compact structural core characteristic of PX domains. Analysis of the backbone dynamics of SNX22 by NMR relaxation measurements revealed that the two walls of the ligand binding cleft undergo internal motions: on the picosecond timescale for the 1/2 loop and on the micro- to millisecond timescale for the loop between the polyproline motif and helix 2. Regions of the SNX22 structure that differ from those of other PX domains include the loop connecting strands 1 and 2 and the loop connecting helices 1 and 2, which appear to be more mobile than corresponding loops in other known structures. The interaction of dibutanoyl-phosphatidylinositol-3-phosphate (dibutanoyl-PtdIns(3)P) with SNX22 was investigated by an NMR titration experiment, which identified the binding site in a basic cleft and indicated that ligand binding leads only to a local structural rearrangement as has been found with other PX domains. Because motions in the loops are damped out when dibutanoyl-PtdIns(3)P binds, entropic effects could contribute to the lower affinity of SNX22 for this ligand compared to other PX domains.

Keywords: domains and motifs; protein trafficking/sorting; NMR spectroscopy

Supplemental material: see www.proteinscience.org

Reprint requests to: John L. Markley, Center for Eukaryotic Structural Genomics, Department of Biochemistry, 433 Babcock Drive, University of Wisconsin-Madison, Madison, WI 53706-1544, USA; e-mail: markley{at}nmrfam.wisc.edu; fax: (608) 262-3759.

Article published online ahead of print. Article and publication date are at http://www.proteinscience.org/cgi/doi/10.1110/ps.072752407.

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