Developmental regulator acting by forming heterodimeric complexes, that sequester CO and CO-like (COL) proteins into non-functional complexes. Engages CO and the transcriptional repressor TPL in a tripartite complex. Involved in the CO-mediated long-day flowering-promotion pathway. Interacts with CO (via B-box) and with TPL (via PFVFL motif). Highly expressed in shoot apical meristems and in vascular tissues of leaves. Also detected in petioles. Circadian-regulation. Peak of expression toward the end of the dark period in both long day and short day photoperiods. The PFVFL motif is required for interaction with TPL.