We clarified this issue by identifying and characterising a predicted Cterminal membrane helix that mediated the restricted subcellular localisations of CaBP7 and CaBP8. Outstanding issues relating to this targeting mechanism concerned firstly whether the putative hydrophobic C-terminal region was indeed a functional TM domain? and secondly, what was the exact topology adopted by both proteins at their target membranes? The C-terminal position of the CaBP7 and CaBP8 TM helix strongly resembles the organisation expected of a classic tailanchored protein. This family of proteins typically have a comparatively large cytosol-oriented N-terminal functional domain a single TM spanning domain and short C-terminal luminal domain. TA proteins can therefore be considered a special class of type-II membrane PF 750 protein and depending on the exact composition of their TM domain are directed either to the endoplasmic reticulum from where they can traffic on to other destinations along the secretory pathway or are immediately inserted into the mitochondrial outer membrane which represents a trafficking endpoint. In co-localisation studies with mitochondrial markers we have ascertained that CaBP7 and CaBP8 do not traffic to these organelles. TA class proteins represent 2.02% of all coding open reading frames in the human genome and have been implicated in important aspects of cell physiology ranging from control of mitochondrial function, apoptosis and intracellular vesicular trafficking. TA proteins lack a signal peptide and since their TM domain only exits the ribosome near Pregnanolone translation termination they are not, in the majority of cases, believed to be co-translationally translocated across the ER membrane in a Sec61 translocon dependent fashion. Various studies have examined the requirement for other protein factors in mediating post-translational insertion of TA proteins into the ER/mitochondrial membranes and at present it appears that at least three distinct pathways are involved in this process, each broadly exhibiting a preference for TM helices of distinct hydrophobicity. We had previously shown that CaBP7 and CaBP8 are membrane associated. In this study we have focused on determining firstly the exact orientation adopted by CaBP7 and CaBP8 at cellular membranes and secondly whether they are processed as TA class proteins. These findings are fundamental as the topologies of CaBP7 and CaBP8 have clear implications for their reported biological activity in the regulation of the essential Golgi trafficking enzyme PI4K which is a cytosolic effector. In order for CaBP7 and CaBP8 to interact with PI4K and respond to fluxes in cytosolic Ca2+ their EF-hand containing Nterminal domains, as described, would need to face the cytosol. Their C-termini could be either luminal or cytosolic.