## 1 Introduction  
The two adhesins of _Pseudomonas putida_, LapA and LapF, are responsible for attachment, initial biofilm formation, and maintaining the structural integrity of mature biofilm. LapA is presumed to be an essential adhesin for attachment and biofilm formation, while LapF is characterised as an adhesin for cell-cell interaction in mature biofilm. However, the role of LapA in biofilm appears to be more critical [1; 2; 3; 4]. The transport and exposure of _Pseudomonas fluorescens_ LapA onto the cell surface have been extensively studied. Due to the similarity of the system to _P. putida_, it can be considered as a model for both bacteria. The LapA of _P. fluorescens_ is transported to the cell surface via a type I secretion system (T1 SS) that consists of three proteins, LapB, LapC and LapE [5]. LapB is expected to recognise the C-terminal signal sequence of LapA. The C-terminus of the protein is transported out of the cell, while the N-terminus remains in the periplasm [6]. LapA remains tethered to the outer membrane via LapE, inducing attachment to biotic and abiotic surfaces [7; 8; 9; 10]. The periplasmic protease LapG releases LapA from LapE after the proteolytic attack of the N-terminus of LapA, thereby reducing the biofilm. Depending on the presence of c-di-GMP, LapD, submerged to the inner membrane, can inhibit the LapG activity by binding to it [11]. Unlike _P. fluorescens_, the _P. putida_ LapA transporter genes are not located consecutively; _lapBC_ (PP_0167 and PP_0166) form an operon, whereas _lapE_ (PP_4519) is located separately. It is important to note that _lapA_ and _lapB_ of _P. putida_ share the same, approximately 1 kbp long, promoter region, with the genes encoded in the opposite direction [12]. The LapAs are large extracellular proteins; in _P. fluorescens_ Pf0-1 and _P. putida_ KT2440 the size of the protein is 519 kDa and 888 kDa, respectively [12]. Based on the similarity of protein sequence, several domains have been found in LapAs that could be involved in its transport and adhesion. While LapA N-terminus remains in the periplasm, the extracellular repeats make up the majority of LapA. Although these repeats are not conserved among LapA homologs, it is suggested that the functionally similar repetitions provide a tubular structure, ensuring the location of the C-terminus far away from the cell [13]. The C-terminus of LapA, the extracellular part of this protein, contains the T1 SS signal sequence required to transport with the LapBCE complex and a repeat-in-toxin (RTX) domain, described as a domain that binds calcium [10; 12; 14]. One particular domain of the C-terminus, the von Willebrand factor A-like domain (vWFa), has our attention. In _P. putida_ strain KT2440, amino acids 8159-8283 of LapA are predicted to form the vWFa-domain based on the Pfam database and _P. fluorescens_ Pf0-1 in 4710-4830, respectively [12]. The vWFa of _P. fluorescens_ LapA is associated with adhesion into hydrophilic surfaces [10], but so far, the function of _P. putida_ vWFa has not been described. The von Willebrand factor (vWF) is a eucaryotic multimeric glycoprotein found in blood plasma, platelets, endothelial cells, and subendothelial cells. Its primary function is to induce the adhesion of platelets and thrombus formation in the case of vascular injury [15]. Several eukaryotic proteins, such as type IV collagen, type VII collagen, cartilage matrix protein, undulin, leukocyte \(\beta\)2 class receptors, complement components C2, B, etc., contain a domain A of vWF (vWFa). These proteins are mostly cell surface proteins or extracellular proteins [16]. In addition to eucaryotic proteins, domain A of the von Willebrand factor is found in several bacterial proteins, especially among extracellular adhesins such as LapA homologs [11]. Hence, domain A is particularly characteristic of extracellular proteins involved in cell adhesion, migration, homing, pattern formation, and signal transduction [16]. Chronic infection of all pathogenic bacteria begins with their attachment to the surface.