This protein family includes PII-interacting protein X (PipX) from Synechococcus elongatus and similar proteins found in cyanobacteria. PipX modulates the expression of genes involved in nitrogen assimilation by interacting with the global nitrogen transcription factor NtcA and the signal transduction protein PII. It is involved in several regulatory pathways [1]. The crystal structure of this protein shows a tudor-like domain (TLD), formed by a highly bent beta-sheet that mediates the interactions with both PII and NtcA, and two C-terminal helices [2].

1: PipX, the coactivator of NtcA, is a global regulator in cyanobacteria. Espinosa J, Rodriguez-Mateos F, Salinas P, Lanza VF, Dixon R, de la Cruz F, Contreras A; Proc Natl Acad Sci U S A. 2014;111:E2423-E2430. PMID:24912181

2: Structural basis for the regulation of NtcA-dependent transcription by proteins PipX and PII. Llacer JL, Espinosa J, Castells MA, Contreras A, Forchhammer K, Rubio V; Proc Natl Acad Sci U S A. 2010;107:15397-15402. PMID:20716687