The IFT A complex is believed to be composed of six components: WDR19/IFT144, IFT140, IFT122, TTC21B/IFT139, WDR35/IFT121 and IFT43 (Piperno et al, 1998; Cole and Snell, 2009; reviewed in Taschner et al, 2012). Each of these proteins was identified as a TULP3-interacting protein in human cells, supporting the notion established in other organisms that they are all components of the IFT A complex (Mukhopadhyay et al, 2010; reviewed in Taschner et al, 2012). The IFT A proteins are large and generally have similar domain organization, consisting of N-terminal WD motifs and C-terminal TPR repeats. These protein interaction domains may help the IFT A complex scaffold recruitment of the IFT B complex, as well as recruit ciliary cargo and motor proteins. Intriguingly, the domain structure of IFT A proteins is similar to that of nucleoporins and coat proteins and it has been suggested that they evolved from a coat protein precursor, consistent with a role in vesicle trafficking (Devos et al, 2004; Jekely and Arendt, 2006).
Details of protein-protein interactions within the IFT A complex are not known, nor are the details of how and where the complex assembles in a human cell.