Dependence of Site-2 Protease Cleavage of ATF6 on Prior Site-1 Protease Digestion Is Determined by the Size of the Luminal Domain of ATF6

Department of Biological Sciences, Columbia University, New York, New York 10027, USA.
Journal of Biological Chemistry (Impact Factor: 4.57). 11/2004; 279(41):43046-51. DOI: 10.1074/jbc.M408466200
Source: PubMed


ATF6 is an endoplasmic reticulum (ER) membrane-anchored transcription factor activated by regulated intramembrane proteolysis in the ER stress response. The release of the cytosolic transcription factor domain of ATF6 requires the sequential processing by the Golgi site-1 and site-2 proteases (S1P and S2P). It has been unclear why S2P proteolysis relies on previous site-1 cleavage. One possibility is that S2P localizes to a different cellular compartment than S1P; however, here we show that S2P localizes to the same compartment as S1P, the cis/medial-Golgi. In addition, we have re-localized S1P and S2P to the ER with brefeldin A and find that the sequential cleavage of ATF6 is reconstituted in the ER. The mapping of the region of ATF6 required for sequential S1P and S2P cleavage showed that short luminal domains resulted in S1P-independent S2P cleavage. The addition of artificial domains onto these short ATF6 luminal domains restored the S1P dependence of S2P cleavage, suggesting that it is the size rather than specific sequences in the luminal domain that determines the S1P dependence of S2P cleavage. These results suggest that the bulky ATF6 luminal domain blocks S2P cleavage and that the role of S1P is to reduce the size of the luminal domain to prepare ATF6 to be an optimal S2P substrate.

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    • "It is interesting to note that bZIP28Δ355 lacks a S1P processing site. Cleavage at the S1P site is usually considered to be a prerequisite for S2P cleavage, which releases the transcriptional component of stress sensor/transducer from the Golgi for relocation to the nucleus (Espenshade et al., 1999; Shen and Prywes, 2004). This implies that S1P cleavage is not required for S2P proteolysis as long as the C-terminal tail of bZIP28 has been removed. "
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    • "ATF6 exists as an ER membrane-resident transcription factor that is anchored by a luminal domain. Accumulation of unfolded proteins triggers transport of vesicles containing ATF6 to the golgi where the luminal domain is sequentially removed by S1P and S2P proteases normally associated with sterol response element binding protein processing (Ye et al., 2000; Shen and Prywes, 2004). The exact mechanisms of proteolytic processing of ATF6 and the nature of luminal-domain sensing of ER stress are not well defined but it is proposed that the unusual structure of the luminal domain contain redox-sensitive disulfide bridges or chaperone interaction domains that monitor the status of the ER (Walter and Ron, 2011). "
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    • "When the ER is overwhelmed by the accumulation of unfolded proteins, BiP preferentially associates with them and dissociates from the three receptor/sensor proteins (Bertolotti et al., 2000). Both IRE1 and PERK, free to homodimerise, get activated by auto-transphosphorylation, whereas ATF6 moves to the Golgi, where it is released after cleavage in the cytosol and translocates into the nucleus (Shen and Prywes, 2004). Although the three UPR branches of the UPR signalling pathway are simultaneously activated upon ER stress, the behaviour of each of these branches varies markedly in time after the onset of the stress (Lin et al., 2007). "
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