We previously identified Scyl1 as a high affinity-binding partner for COPI coats, and determined a role for Scyl1 as a regulatory component in COPI-mediated retrograde trafficking of KDEL receptor 
. Here we demonstrate that depletion of Scyl1 results in an expanded but intact Golgi apparatus. A similar Golgi expansion phenotype was seen in NRK cells following knock down of TMF/ARA160, a Rab6-binding protein that localizes to the Golgi 
. In this case, there was a loss of the tight, juxtanuclear Golgi staining with a modest dispersal of the Golgi, which appeared to remain intact 
. Combined with our previous work, our current study supports the hypothesis that a decrease in retrograde flux of KDEL receptor-positive COPI vesicles following Scyl1 knock down increases the size of the Golgi, similar to the increase in size observed following an increase in anterograde trafficking flux upon VSVG overexpression 
. It is known that COPI function is essential in cells, and that lack of εCOP leads to cell death, while knock down of βCOP disrupts Golgi homeostasis 
. It is less clear how depletion of COPI accessory factors influence cellular function. Loss of function of Scyl1 results in neurodegeneration of motor neurons in mdf mice, resulting in a recessive spinocerebellar ataxic disorder 
. This suggests that loss of a ubiquitously expressed regulatory component of the COPI pathway does not necessarily result in death, but rather selective degeneration of a subset of vulnerable motor neurons that likely are more dependent on the COPI pathway than other cells, perhaps due to their large size and high degree of polarization 
. Future studies will examine the role of Scyl1 in neuronal Golgi function.
While Scyl1 displays some biochemical properties of a golgin, important differences exist. One property shared by many golgins is that they interact with Rab proteins. We have thus far been unable to detect a convincing interaction of Scyl1 with Rabs. However, previous studies identified the protein Scyl1 binding partner 1 (Scyl1BP1) based on its binding to Scyl1 
. Recent evidence demonstrates that Scyl1BP1 (also known as Golgi-associated Rab-binding protein or GORAB) binds Rab6 in a GTP-dependant manner and localizes to the Golgi, prompting the authors to classify GORAB as a golgin 
. Interestingly, when mutated, GORAB results in Gerodermia Osteodysplastica (GO) in humans 
. Thus a Scyl1/GORAB/Rab6 complex may exist. However, whereas Scyl1 is preferentially localized to the cis-Golgi, GORAB and Rab6 are polarized to the TGN 
. Moreover, GO patients suffer from degeneration of bone and skin but do not appear to display neurological deficits or neurodegeneration 
. Thus, whether or not Scyl1 has direct or indirect interactions with Rabs is at present unknown.
We did detect an interaction of Scyl1 with 58K and we demonstrate that an intact golgin network is required for Scyl1 association with the Golgi. 58K is a Golgi localized protein that appears to mediate an interface between the Golgi and the vimentin intermediate filament cytoskeleton 
. As for Scyl1, 58K is tightly associated with Golgi membranes but cycles on and off the Golgi 
. Moreover, like Scyl1 58K remains associated with Golgi fragments during microtubule disruption and is not released into cytosol during BFA treatment 
. Thus, it is possible that 58K contributes to Scyl1 recruitment to the Golgi. Since 58K and βCOP bind to predominantly non-overlapping sites on Scyl1, Scyl1 could be at an interface between 58K, the golgin network and COPI coats. Disruption of the golgin network or knock down of Scyl1 would thus disrupt retrograde COPI-mediated trafficking.
Another important property of golgins is that in many cases, for example with GM130 and p115, their knock down results in a fragmented Golgi. In contrast, Scyl1 knock down cells maintain an intact and polarized Golgi as shown from the polarized relationship between GM130 and TGN46, as well as by EM analysis of Scyl1-depleted cells. Thus, unlike GM130, Scyl1 is unlikely to have a role in tethering adjacent Golgi cisternae 
. It is known that p115, Giantin and GM130 function as tethers to mediate vesicle-docking events at the cis-Golgi. It is hypothesized that only incoming vesicles are affected by these docking events, and that retrograde trafficking should be relatively unperturbed 
. However, it has been shown that p115-positive COPI vesicles contain KDEL receptor 
, and we have shown that knock down of Scyl1 results in a defect of trafficking of the KDEL receptor from the cis-Golgi to the ER 
. Moreover, knock down of p115 disrupts the localization of Scyl1 on the Golgi. Therefore, it is possible that Scyl1 uses its 58K binding/golgin network association to position itself to regulate KDEL receptor trafficking events more directly, possibly through the modulation of KDEL receptor phosphorylation, a state that is known to be critical for KDEL receptor inclusion in COPI retrograde directed vesicles 
. Although the N-terminal kinase-like domain of Scyl1 almost certainly lacks catalytic activity, it could function as a kinase competitor or substrate trap to regulate the phosphorylation status of proteins. In conclusion, our data demonstrate that Scyl1 has a unique position between the golgin network and the COPI machinery and that it regulates Golgi homeostasis, most likely via the modulation of retrograde membrane trafficking events.