Physical and Functional Interaction between Myeloid Cell Leukemia 1 Protein (MCL1) and Fortilin

Abstract

Myeloid cell leukemia 1 protein (MCL1) is an antiapoptotic protein that is structurally related to Bcl-2. Unlike other Bcl-2 family proteins that are constitutively expressed, MCL1 is inducibly expressed in cells that are recently exposed to growth and differentiation stimuli. Here, we report the identification of fortilin as a novel MCL1-interacting protein by screening of a yeast two-hybrid library with MCL1 as bait. Fortilin specifically interacted with MCL1 both in vitro and in vivo. The intracellular localization of fortilin was predominantly nuclear and identical to that of MCL1, as shown by immunostaining and confocal microscopy analysis. Fortilin, like MCL1, was rapidly inducible in serum-stimulated human aortic vascular smooth muscle cells. Although the depletion of intracellular fortilin by small interfering RNA (siRNA) against fortilin (siRNAfortilin) did not affect intracellular MCL1 level, the depletion of intracellular MCL1 by siRNA-MCL1 was associated with the significant reduction of the fortilin protein level, without affecting the fortilin transcript numbers. In addition, a pulse-chase experiment showed that the depletion of MCL1 by siRNA-MCL1 was associated with the rapid degradation of fortilin protein, which was found quite stable in the presence of MCL1. Furthermore, the half-life of fortilin R21A , a point mutant of fortilin lacking the binding to MCL1, was significantly shorter than that of wild-type fortilin as shown by a pulse-chase experiment. These data suggest that MCL1, in addition to being an anti-apoptotic molecule, serves as a chaperone of fortilin, binding and stabilizing fortilin in vivo. Taken together with our previous observation that fortilin overexpression prevents cells from undergoing apoptosis (Li, F., Zhang, D., and Fujise, K. (2001) J. Biol. Chem. 276, 47542-47549), it is likely that MCL1, an anti-apoptotic protein inducible by growth and differentiation stimuli, stabilizes another anti-apoptotic protein fortilin maximizing the prosurvival environment in cells.

Roles of the translationally controlled tumour protein (TCTP) and the double-stranded RNA-dependent protein kinase, PKR, in cellular stress responses

“…This situation is strikingly similar to the one described here for TCTP. Interestingly, Mcl-1 has been described as a protein stabilizing TCTP (Zhang et al, 2002) and vice versa (Liu et al, 2005). These results are compatible with a model according to which activation of PKR by proapoptotic stimuli results in an increase in eIF2a phosphorylation and downregulation of antiapoptotic proteins (Scheuner et al, 2006), such as TCTP (Figure 7) or Mcl-1 (Fritsch et al, 2007).…”

“…Our observations on both the pattern of PKR-dependent downregulation of TCTP under Ca þ þ -stress conditions and on the cytoprotective function of TCTP 6) also suggest that the protein has broader antiapoptotic effects. Several reports support the view that TCTP undergoes direct interactions with the apoptotic machinery (Zhang et al, 2002;Liu et al, 2005;Yang et al, 2005;Susini et al, 2008) The antiapoptotic protein Bcl-xL, with which TCTP interacts, is involved in maintaining Ca þ þ homeostasis in the ER (Xu et al, 2005) and it is, therefore, possible that involvement of TCTP in the cell's response to Ca þ þ stress is based on this interaction. It was recently reported that translational downregulation of another antiapoptotic protein, Mcl-1, is an important permissive step in the apoptotic events induced by cell-stress reagents such as tunicamycin, thapsigargin and arsenite (Fritsch et al, 2007).…”

“…Recent reports described the importance of TCTP for the reprogramming of somaticcell nuclei after transfer into oocytes or stem cells (Koziol et al, 2007;Tani et al, 2007). TCTP (fortilin) has been characterized as an antiapoptotic protein (Li et al, 2001); it interacts with other antiapoptotic proteins, Mcl-1 (Zhang et al, 2002;Liu et al, 2005) and Bcl-xL . This cytoprotective role of TCTP is consistent with the recently reported antioxidant function of filarial TCTP (Gnanasekar and Ramaswamy, 2007), and with observations that TCTP levels are highly regulated in various cell-stress conditions (Schmidt et al, 2007).…”

The Translational Controlled Tumour Protein TCTP: Biological Functions and Regulation

“…Mcl-1 (Liu et al 2005;Zhang et al 2002) and Bcl-XL ). In the case of Mcl-1, it was reported that TCTP stabilises Mcl-1 (Liu et al 2005) and vice versa (Zhang et al 2002), but another study also showed that both proteins are able to exert their anti-apoptotic activity independently of each other . Recently, the interaction of TCTP with Bcl-XL was investigated in more detail (Thebault et al 2016).…”

“…There are a few reports, which indicate that in specific instances TCTP protein may be subject to regulated degradation. Two studies described the ability of another anti-apoptotic protein, Mcl-1 (Zhang et al 2002), and heat-shock protein Hsp27 (Baylot et al 2012) to stabilise the TCTP protein, which implies that it may be destabilised if those proteins are absent. Kubiak et al reported that TCTP, a protein involved in maintaining the integrity of the mitotic spindle, is partially degraded during mitotic exit (Kubiak et al 2008).…”