The biogenesis of most spliceosomal small nuclear ribonucleoproteins (snRNPs) is complicated and requires both cytoplasmic and nuclear maturation steps [1–3]. For example, the spliceosomal small nuclear RNAs (snRNAs) of Ul, U2, U4 and U5 snRNPs are synthesized by RNA polymerase II and may traffic through specific subnuclear domains before being exported to the cytoplasm [1, 3]. In the cytoplasm, a septet of Sm proteins (B/B', Dl, D2, D3, E, F, G) binds the Sm motif of the snRNA under the control of the Survival of Motor Neurons (SMN) protein complex. Mutations in the SMN protein cause the neurodegenerative disorder Spinal Muscular Atrophy [4, 5]. After the Sm core has been assembled onto the snRNA, the snRNA is subject to further processing, followed by import back into the nucleus; again with the help of the SMN complex [1, 5–8].
Upon nuclear re-entry, newly assembled Ul, U2, U4 and U5 snRNPs first localize to a subnuclear domain known as the Cajal body . In the Cajal body (CB), the snRNA component of the snRNP is subjected to pseudouridine-base and 2'-0-methyl sugar-modifications that are guided by small CB-specific RNAs (scaRNAs) [10–12]. These modifications are crucial for proper pre-mRNA splicing in vivo . After their modification in the CB, snRNPs localize to speckles, where they are stored, or perichromatin fibrils, where splicing occurs concurrently with transcription . Unlike Ul, U2, U4 and U5 snRNAs, maturation of the RNA polymerase III-transcribed U6 snRNA does not include a cytoplasmic phase and may take place in the nucleolus and the CB . The U7 snRNP, which is required for histone pre-mRNA 3'-end processing [15–17], is assembled in manner similar to that observed for Ul, U2, U4 and U5 snRNPs, including a cytoplasmic phase . However, U7 snRNA has a noncanonical Sm binding site and thus recruits a different type of Sm core compared to that which binds Ul, U2, U4 and U5 snRNA [19, 20]. Like the spliceosomal snRNPs, the U7 snRNP is enriched within CBs [19, 21]. CBs have been shown to move in an ATP dependent manner [22, 23] as well as associate with various gene loci, including snRNA and histone gene clusters . Consequently, CBs may provide a platform upon which a feedback regulatory mechanism for snRNP and histone biogenesis takes place .
The mechanisms by which snRNPs are targeted to and released from the CB are unknown. One possibility is that a factor within the CB interacts with nascent snRNPs and facilitates their modification. Another factor may displace snRNPs from the CB, allowing for their subsequent localization in speckles and perichromatin fibrils. The CB marker protein coilin may play a role in the targeting of snRNPs to CBs. Removal of coilin in Xenopus by immunodepletion decreases snRNP levels in the amphibian equivalent of the CB . Characterization of coilin knockout mice has revealed that they are viable on an outbred mouse strain, but have significant viability and fertility defects on inbred strains  (Greg Matera, personal communication). Cell lines derived from coilin knockout mice lack canonical CBs in which snRNPs are enriched . However, add-back experiments demonstrate that typical CBs, containing snRNPs, can be reformed upon the addition of coilin . Furthermore, coilin can interact directly with several Sm proteins of snRNPs  (our unpublished observations). Taken together, these data indicate that, while not an essential protein, coilin is important for proper CB formation. Functional CBs may allow for the efficient coordination of the nuclear steps of snRNP biogenesis.
Another protein that requires coilin for its localization to CBs is SMN [27–30]. Coilin directly interacts with SMN via several arginine/glycine (RG) dipeptide repeats in coilin . The arginines within this RG box motif are symmetrically dimethylated [29, 30], resulting in an increased affinity for SMN [31, 32]. In most cell lines and tissues, SMN localizes to the cytoplasm and CBs . However, SMN in some cell lines and fetal tissue localizes to discrete nuclear structures termed "gems" (for Gemini of Cajal bodies) [34, 35]. The presence of gems correlates with a decrease in coilin methylation [29, 30]. Although the nuclear role of SMN is not well understood, it is possible that SMN escorts nascent snRNPs to the CB [6, 8]. Interestingly, coilin can compete with Sm proteins for binding sites on SMN , indicating that locally high concentrations of coilin within the CB may serve to displace snRNPs from the SMN complex. The interplay between SMN, snRNPs and coilin may therefore regulate snRNP accumulation within the CB. We set out to determine if we could identify factors that control the departure of nascent snRNPs from the CB.
In this report, we demonstrate that an isoform of the human mitochondrial Tim50, Tim50a, localizes to speckles and interacts with snRNPs, SMN and coilin. Human Tim50, which has phosphatase activity, is a component of the mitochondrial translocator and regulates mitochondrial integrity and cell death . The protein sequence of Tim50 and Tim50a are identical, with the important exception that Tim50a contains an alternative translational start sequence that adds 103 aa to its N-terminus relative to Tim50. Competition binding experiments show that SmB' does not compete with SMN for Tim50a binding sites, but SmB' does reduce coilin interaction with Tim50a. Furthermore, SMN and coilin compete for Tim50a binding sites and Tim50a forms a more efficient complex with snRNPs in vivo compared to Tim50. Based on these results, we propose that Tim50a is involved in the regulation of snRNP biogenesis and possibly the activity of nuclear SMN.