Role of Prodomain in Importin-mediated Nuclear Localization and Activation of Caspase-2

Abstract

Caspase-2 is unique among mammalian caspases because it localizes to the nucleus in a prodomain-dependent manner. The caspase-2 prodomain also regulates caspase-2 activity via a caspase recruitment domain that mediates oligomerization of procaspase-2 molecules and their subsequent autoactivation. In this study we sought to map specific functional regions in the caspase-2 prodomain that regulate its nuclear transport and also its activation. Our data indicate that caspase-2 contains a classical nuclear localization signal (NLS) at the C terminus of the prodomain which is recognized by the importin ␣/␤ heterodimer. The mutation of a conserved Lys residue in the NLS abolishes nuclear localization of caspase-2 and binding to the importin ␣/␤ heterodimer. Although caspase-2 is imported into the nucleus, mutants lacking the NLS were still capable of inducing apoptosis upon overexpression in transfected cells. We define a region in the prodomain that regulates the ability of caspase-2 to form dot-and filament-like structures when ectopically expressed, which in turn promotes cell killing. Our data provides a mechanism for caspase-2 nuclear import and demonstrate that association of procaspase-2 into higher order structures, rather than its nuclear localization, is required for caspase-2 activation and its ability to induce apoptosis.

Caspase function in programmed cell death

“…101 An unusual feature of caspase-2 is its localisation to the nucleus and the Golgi in addition to cytosol, although its protein targets in these compartments remain obscure. [102][103][104][105] In some cells, caspase-2 was demonstrated to be required for mitochondrial outer membrane permeabilization (MOMP) and the release of apoptogenic factors in response to DNAdamaging agents. 106 Caspase-2 has also been shown to be necessary for TRAIL-mediated, 107,108 heat shock (HS)-induced apoptosis, 109,110 as well as oocyte cell death.…”

Caspase-2 as a tumour suppressor

“…A unique feature of caspase-2 is its nuclear localization, a property not shared by other caspases. 7,8 Caspase-2 has been implicated in apoptosis induced by multiple intrinsic and extrinsic stimuli including DNA damage, reactive oxygen species (ROS) and cytoskeletal disruption ( Figure 1). 9 However, as discussed below, deletion of caspase-2 in mice does not lead to a phenotype that would support a broad function for this caspase in apoptosis.…”

“…45 Caspase-2 is also actively imported into the nucleus via a nuclear localization sequence (NLS) located in the prodomain. 7,8 Caspase-2 activation has been shown to be regulated by phosphorylation mediated by various kinases, including calcium/calmodulin-dependent protein kinase II (Ser164), 47 protein kinase CK2 (Ser157) 48 and cyclin-dependent kinase 1 (Ser340). 49 …”

Real time with Caspase-2

“…This is in contrast to numerous reports that show caspase-2 localized to the nucleus and suggestions that it functions from this location. 7,8 We observed caspase-2 activation in the cytosol in response to various apoptotic stimuli including metabolic stress, cytoskeletal disruption, DNA damage and most impressively by heat stress. The induction of caspase-2 by heat stress suggests a connection to the heat shock response.…”