The basic enzymatic reactions catalyzed by PARP1 involve transferring ADPr from NAD to either a protein acceptor or to an existing poly(ADP-ribose) (pADPr) chain 
. More than thirty pADPr acceptor proteins have been identified. However, multiple observations in vivo
and in vitro
have confirmed that the main acceptor of nuclear pADPr in vivo
is PARP1 protein itself (reviewed in 
). This suggests that PARP1 automodification plays important roles in the regulation of nuclear functions. To understand these roles, the present study proposes to elucidate the mechanisms of such automodified PARP1 action on nuclear processes.
Most PARP1 protein binds to chromatin and accumulates in nucleoli 
. Previously, we showed that local activation of transcription in specific domains of chromatin always correlates with local activation of PARP1 protein enzymatic reaction followed by automodification of PARP1 protein within the same chromatin domains 
. The automodification of PARP1 causes it to dissociate from the chromatin template and lose its ability to synthesize pADPr 
. This event, however, allows either the automodified PARP1, or free pADPr, to gain another special biological activity by specifically enabling either one to interact non-covalently with a broad class of nuclear proteins which bear a conserved domain recognizing pADPr 
. Thus, automodified PARP1 which accumulates around a particular locus could serve as a local signal for binding and transportation of those proteins, as well as for modification of their biochemical properties. To date, however, no one has shown the functional significance of those interactions in vivo
Poly(ADP-Ribose)Glycohydrolase (PARG) protein is the only known catalyst of pADPr degradation in vivo 
. Recently, we demonstrated that loss of PARG reduces overall PARP1 activity by trapping a large fraction of the PARP1 protein in the automodified state, which is removed from active chromatin and then accumulates in specific nucleoplasmic organelles 
. These findings raise the possibility that automodified PARP1 targets from chromatin into these organelles and that, consequently, automodified PARP1 could serve as a shuttle to deliver other proteins interacting with pADPr into these organelles. In this paper, we show that those organelles are Cajal bodies (CBs). CBs (also known as “Coiled bodies”) are spherical sub-organelles found in the nucleus of proliferative or metabolically active cells in plant, yeast, and animals. CBs are possible sites of assembly or modification of the transcription machinery of the nucleus 
. Since PARP1 protein also controls transcription and nucleoli and since automodified PARP1 is targeted into CB, PARP1 could be an important link in this process. Moreover, CBs are often seen attached to the nucleolus and share many nucleolar protein components, such as protein Fibrillarin 
and p80 protein coilin which is a common specific marker of CBs 
that we have used in our experiments.
Here, we show that PARP1 protein interacts with key components of CBs and that the presence of pADPr is critical for their delivery to CB. Specifically, our data show that chromosomal PARP1 molecules become activated by developmental cues, such as ecdysteroid signaling, and are automodified by pADPr. Automodified PARP1 then binds protein components of nucleoli and chromatin. Following that, automodified PARP1 serves as a “shuttle” for protein delivery to Cajal bodies for recycling, thus contributing to protein trafficking through CBs and to the overall stability of Cajal body.