2 International Journal of Cell Biology PLD Fusion MIB MFN1 s-OPA1 I-OPA1 MFN2 MTP18 Prohibitin DRP1 p CAMK1α PKA FIS1 MIEF DRP1 LETM1 p p Ca 2+ Calcineurin MFF GDAP1 (a) Fission Endophilin 1 (b) Figure 1: The dynamic nature of mitochondrial shape. (a) Main proteins involved in mitochondrial shape changes are depicted. In fused unopposed condition, DRP1 is phosphorylated and sequestered in the cytoplasm. Once dephosphorylated, it is recruited to the OMM where it oligomerizes and interacts with FIS1, MFF, or MIEF inducing constriction of membranes and, eventually, ﬁssion of mitochondria. MFNs homo- and heterooligomerization on the OMM and oligomerization between long and short isoform of Opa1 on the IMM control fusion of mitochondrial membranes. Additional proteins aﬀecting mitochondrial shape are also shown. (b) Mitochondrial morphology in Jurkat cells overexpressing yellow ﬂuorescent protein targeted to mitochondria. The upper panel shows a network of elongated and interconnected mitochondria. In the lower panel, mitochondria appear fragmented (Scale bar: 5 μm). In summary, the pleiotropic mitofusins (MFN1 and MFN2) and OPA1 are the main regulators of the mitochon- dria fusion machinery. Although steps forward have been made, some points of this mechanism still remain to be clariﬁed; in particular, how the IMM fuses and how its fusion is coordinated with events of the OMM fusion. 1.2. Mitochondrial Fission. The ﬁssion machinery is based on DRP1, FIS1, MFF, and MIEF1. DRP1 is a large GTPase protein [13], which in conditions of unopposed fusion is cytosolic and after dephosphorylation by calcineurin is recruited on mitochondria [14], where it oligomerizes and interacts with its putative adaptors on the OMM (FIS1, MFF, and MIEF1), forming ring-shaped structures and inducing mitochondrial constriction and ﬁssion [15]. Endophilin B1 [16], MTP18 [17], MIB [18], and GDAP1 [19] have been described, moreover, among the ﬁssion components. The scenario of mitochondrial ﬁssion is becoming more complex, taking into account the diﬀerent roles proposed for MFF, MIEF, and FIS1. MFF has recently been shown to be an adaptor of DRP1 on the OMM [20], whereas the binding of MIEF to DRP1 on the OMM inhibits the GTPase function of DRP1 and its proﬁssion activity [21]. On the other hand, FIS1, that initially was considered the only DRP1 receptor at the OMM, has now been proposed to exert its proﬁssion role by interacting and sequestering MIEF, thus allowing DRP1 to mediate the constriction and ﬁssion of the membrane [21]. It has also been suggested that mitochondrial ﬁssion events predominantly occur at the contact sites between ER and mitochondria, speciﬁcally where some ER tubules cross over and wrap around mitochondria. Interestingly, DRP1 and MFF localize at these contact sites [22]. New studies that will address the biochemical mechanism, by which the ER participates in mitochondrial division, placing this observation in the overall picture of mitochondrial division, will be of great interest in the ﬁeld. In summary, the ﬁssion machinery depends on the activation and translocation of DRP1 to the OMM, where it interacts with some other components, not clearly deﬁned, that take part to the modulation of mitochondrial fragmen- tation. 1.3. More than Just Morphology: The Intimate Connection with Physiological Functions. Changes in mitochondrial morphol- ogy have been related to alterations in mitochondrial func- tion, transport, location, and quality control. Mitochondrial